Do you need it now? Or can you wait?
If you can wait, think about it this way. When the 6D (or whatever, low-cost FF) is is introduced, there's going to be a lot more FF cameras going around. So as long as it's under $2500, some people would be 'upgrading' their old 5D2s to the 6D, depending on which features they do and don't want (ie, if the AF in the new body really is as good as the 7D, some people might ditch their 5D2 just for that, if the sensor is roughly comparable, even if it means less other features).
In short, I'm guessing that when the 6D drops, the 5D2 will get cheaper.
At that time you will need to decide between the 6D and 5D2, depending on whatever specs you want and the cash you have.
So the decision is
- buy a 5D2 now, and lose a bit on the re-sale value, but you get to use it now.
- or keep the crop for now, wait until the 6D drops, whenever that may be, could be photokina next week, could be this time next year, and decide between 5D2 and 6D then.
(personally, i'd be waiting, but i've got enough toys to play with for years without buying new ones. If you need it soon, then it's going to be 5D2)