First, the disclaimer: although I do have long-distance collaborations, I've not yet done much real-time serious mathematics. The reasons for this are many and varied, but one relevant one is simply that my speed-of-thought is actually much slower than (I suspect) many people's so even in short-distance collaborations the "together time" is spent in reporting rather than brain-storming and that's a bit different.
That said, "doing maths online" is a bit of a pet project of mine at the moment, so here's some thoughts.
Use a wiki. Instiki, of course, as it's the only one with decent maths support. This isn't for the actual dialogue, but given that the time is going to be precious, it will be useful to "set the agenda" beforehand and "take minutes" afterwards.
For the actual collaboration, I'd recommend jarnal. It has a client-server part so you can set up a "virtual whiteboard" for collaborating. Of course, you'd need to add the voice bit on top (telephone, per chance?). With a graphics tablet, I'd think that this would work just fine. I've used jarnal for writing in lectures and have found it very easy to use. Writing on a tablet instead of the screen quickly becomes second nature as well. (For the record, I use a Wacom Bamboo Fun tablet which I picked up in the UK for around 30 quid and I find it works just fine.) Jarnal is also cross-platform (written in java) so there's no worry about different operating systems not supporting it. Note that using something like Jarnal has the distinct advantage over webcam+whiteboard that the session is saved automatically.
If the emphasis is less on real time, I would recommend a Mathematics-enabled forum (I happen to know one I could let you have at a very reasonable price, no obvious damage, no income tax, no VAT, ...). It is almost real-time, when necessary one can take the time to compose a longer answer, and the back-and-forth is recorded as well. Again, this probably reflects the fact that my "speed of thought" is slower than most, so I like to be able to read what the other person has written and ponder it a little before replying.
I suspect that a good collaboration would use something akin to all three of those: the real-time for the brainstorming, the forum for the more thoughtful discussions, and the wiki for recording the bits that stand the test of time.