The obvious (and best, I think) answer is to wait, since you're asking people to compare a 5D2 with an unknown.
I'm considering the same thing. Of course, you might be in a much more time-sensitive position than I. Fact is, I don't NEED a new camera.
But, in the sport of these types of discussions, I'll give you some thoughts that I've considered.
Do you use Magic Lantern?
Key argument for me in favor of 5D2: If you use Magic Lantern and rely on its peaking and histogram functions in video a lot (and perhaps its timelapse and myriad other functions), then I think this argues in favor of the 5D2. Magic Lantern is available for the 5D2 today and is unlikely to be available for the new camera, if at all, for another year (just speculating here) after release. Some might say to wait to see if Canon puts any of these features on the new camera. I simply don't see that happening, given Canon's track record.
Do you struggle against moire/aliasing, or have you adapted your shooting style to minimize it to an acceptable level for you?
Key argument for waiting: The new FF camera MIGHT have some of the improvements of the 5D3 with regard to moire/aliasing. My wife has a 5D3 that she lets me use from time to time. The lack of moire/aliasing is really nice and the image is nice to work with in post. This is the only thing that has really caused me to wait, other than simply not wanting to spend more money on a camera. Then again, you might wait and find the new camera's image is essentially the same as the 5D2. It isn't clear that anything will be lost, since the price is rumored to be about the same as the current 5D2 and there are so many 5D2's out there that you should be able to pick one up used, should the new FF lack some features that you really want/need.
Heck the new FF camera MIGHT even have more apparent sharpness or resolution than the 5D3. But, I find this possibility far less likely. If it did, Canon would have a hard time keeping them on the shelf.