While it seems a bit late for the OP, I'd like to add some remarks that might be helpful to the subsequent readers.
Having completed my PhD in Combinatorics, I seem to be inevitably converting to Computational Biology. Let me point out some of the most significant aspects that I have encountered:
The letters of recommendation. Although mentioned by Deane Yang, I feel this was not mentioned strongly enough. I find this to be a major obstacle to my future in computational biology. I have lots of people who would be willing to vouch for me for my expertise in combinatorics, but very few who would vouch for my expertise in computational biology. Those who can vouch for me are only able to make limited comments due to only working in the area a short time. [PS. One tip -- make sure the referees in your previous field have some idea of the significance of your work in the new field (thereby reducing the problem raised by Deane Yang)]
Lack of publications. And moreover, the overall lack of relevant brownie points -- e.g. I've refereed papers in combinatorics, I'm a member of the AMS and other societies, and so on, which are not very relevant.
It is multidisciplinary. So most people who enter this area have a PhD in Biology, Computer Science, Mathematics, Statistics, etc., but not in Computational Biology itself. So most people are in the same boat.
These are neighbouring fields. The advantages of this have already been discussed.
There is significantly more funding in computational biology. This is sheer numbers -- there are more jobs available, so they are easier to get.
There are real world applications. It makes it much easier to argue that this research is worth funding (thinking research grants).
Finally, one tip: try not to "switch" fields, but gradually change from one to another.