Are the block chains actually incompatible yet, or is it just that the two halves of the network can't communicate with each other? If they're not actually incompatible - and I can't see any reason why they should be yet because we haven't reached the 20055 threshhold - then any exchange accepting transactions on the half with the smaller hash rate (and it looks like they are on that half) is risking becoming the victim of a double-spend attack by someone that manages to transfer blocks across.