I was in the same boat a few weeks ago, wanting to go full frame after having a 300d/30d/ 7d. I decided on a used (6mth old, 1200 actuations) 5d mk2, I figured that if I preferred the 7d crop camera then I'd be able to sell the 5d mk 2 on easilly enough. A few weeks on and I prefer the 5d mk2, my wide lenses just seem right when combined with a full frame sensor, and I'll keep the 7d primarily for combining the crop factor with my longer lens.
I suppose I was lucky (or planning ahead) as the only EF-S lens I have is the 60mm macro (gave a 15-55 kit lens away with the 300d to a friend), and I'll keep that for use with the 7D. If all of your lenses are EF-S then it's a much more difficult / expensive proposition.