I'm a mathematician. I want to be able to read recent ArXiv postings on high energy physics theory (String theory) (and perhaps be able to do research). I want to understand compactifications, Dualities, Dbranes, Mbranes etc. What's the easiest way to do so provided I have the necessary knowledge in algebraic geometry, algebraic topology, analysis and differential geometry?

Many string theorists would like to know more algebraic geometry. There are a few of us who know algebraic geometry at a pretty high level (not me) but many more who would like to learn more and feel it would help with their research but find the literature very difficult. I think the optimal solution would be to find such a string theorist and agree that you will teach them algebraic geometry if they will teach you string theory. 


I mean, if you are really trying to understand String Theory, then you're going to have to become fluent in Classical Mechanics, Quantum Mechanics, Quantum Field Theory, and General Relativity first... otherwise the papers are going to be unmotivated and you won't understand the linguistics and you won't know how the results connect to the universe (i.e. they're more than just a sequence of symbols which we call math). That being said, assuming CM/QM/QFT/GR are under the belt, the best place to start is GreenWittenSchwarz's (GWS) Superstring Theory, followed by skimming Polchinski's String Theory. This is supported by my string theory professor when I took it a while ago, Petr Horava (discoverer of Dbranes). From here you can supplement other notes and papers. In Vol.1 of GWS, chapter 2/3 will explain the bosonic string theory (i.e. ignoring fermions) and BRST quantization, which leads to a critical dimension $D=26$. Then chapter 4 will fix this with supersymmetry (i.e. putting back in the fermions), leading to the actual critical dimension $D=10$. After this, gauge anomalies and compactification and dualities and Dbranes can start being assessed. 


Since I am a mathematician and also spent quite efforts on learning string theory, etc., let me add some comments. I agree with David Roberts suggesting this (published book is a little more complete, but not essentially), I partly agree with Chris Gerig "This is more just QFT, and would be a good mathematical perspective after you understand the physics of QFT / String Theory..." I would say that this more concerns lectures by mathematicians: Deligne, Kazhdan, Bernstein, which I would suggest to skip at first reading. And just look at physicits lectures: Faddeev, Gawedzki (fall semester) and Witten, D'Hoker (spring semester) "... , in particular I don't think it help for the papers that get posted on arXiv hepph and hepth." Well, yes, this volume does not cover most interesting developments made in 90ies, but nevertheless as some basics sources, it should be Okay. Let me also agree with Chris Gerig "I mean, if you are really trying to understand String Theory, then you're going to have to become fluent in Classical Mechanics, Quantum Mechanics, Quantum Field Theory, and General Relativity first... " The way which many people in Russia are doing this  is via volumes by Landua Lifshitz. Let me say that volumes 13 (Classical mechanics, Field theory(Classical electrodynamics and General relativity), Quantum mechanics) are quite accessible for mathematicians, even for last years undergrads. But this does not contain Feynman path integral. You may look at Feynman, R.P. and Hibbs, A.R. Quantum Mechanics and Path Integrals. Also LL does not include quantum field theory. You may look at Ramond's short book. And IAS volume discussed above. You may also look at Igor Dolgachev's (mathematician) "Introduction to physics" http://www.math.lsa.umich.edu/~idolga/lecturenotes.html Another question it might be worth to figure out  what aim you are setting for yourself. To become a physicist? Let me tell the story  a friend of mine started as a physicist, but later turned to mathematics, I asked him why ? (cause he is really smart and surely had good perspectives). He answered: "you know in physics 1+2+3+4+.... = 1/12, can you live with this ? Me not." Another story about I. Gelfand who being at Rutgers decided to learn some physics, it is started Okay, but at some point, physicist said "here we divide by the volume of the diffeomorphism group" (you always do it in Faddeev'sPopov approach), after that Gelfand stopped this. (The story from my friend who was Gelfand's student and was personally there). I mean for a mathematician absence of proof/(clear understanding) is like a teeth pain, but true physicists will not even observe a problem :) So there is certain cultural and mental difference, and should choose what is more suitable for you. However, my strong feeling is that mathematical community MUST somehow "learn/absorb/rework/rethink" ideas of QFT and string theory. There are certain important tools and ideas which are now hidden in some physical language and sometimes looks as trick, heuristics, etc..., but should be cleared out, polished, placed in the right position of our mathematical knowledge. We are at certain point where the part of math. community and hepth community are quite close to each other, this will not be forever. So it is important not to loose a chance of gaining physical "intuition" and making from it mathematical theory. Let me speculate a little on the possible place of physical ideas in math. It seems to me they are to certain extent "differential geometry of specific infinitedimensional manifolds". I mean typically in physics we consider the space of all smooth maps from one manifold to another. We write a kind of differential form on this space and integrate it. The problem is that such integration is illdefined business, however it somehow works. I think that the manifolds we work are not some abstract infinitedimensional manifolds, but we should take into account that we consider the space of maps from one finitedim manifold to another finitedim  an this will lead to certain "semiinfinite" structures. Like vertex operator algebras more or less are loop algebras of finitedimensional Lie algebras. To certain extent these ideas kind be made quite precise in topological quantum fields theories see e.g. this discussion: Doing geometry using Feynman Path Integral? 


You could do worse than starting with this book: http://books.google.com.au/books/about/Quantum_Fields_and_Strings.html?id=ecruIiTk05EC&redir_esc=y. 


the most basic book I know of is Enumerative Geometry and String Theory by Sheldon Katz. but of course it doesn't even begin to scratch the surface of the topics you (and not only you) want to understand. 


I have the following book and deals with all the topics you mentioned in addition to Conformal Field Theory for a layman's perspective. http://books.google.com/books/about/String_Theory_Demystified.html?id=S4JyPgw4ZlAC 

