Tough question... really depends on your work climate.
If your stock isn't that great, wait until they bring you on board the other projects. Maybe even after you've been at it for a bit. If you're the golden boy, you might be able to ask once the request is made and pending but before you actually move. I generally wouldn't ask before you actually have confirmation of the extra responsibility, because you want your position to have stabilized (so you can point to things and say "See? I do this and this and this now").
You also have to think of the possibility and consequences of their saying no.
Don't get me wrong, I think that if your responsibilities are being expanded so much you should absolutely ask. But if you're on shakey ground, or the company generally doesn't issue raises outside regular processes, the safest bet may be to push it back to your next annual review.