Well, if you offered me a brand new 7D in exchange for my two year old 5D2 I'd refuse it - not too much of a surprise, perhaps, given the difference in retail price.
No doubt the 5D Mark III will be a better camera but whether that difference will warrant the significant difference there will be in price, money that could be spent on EF glass, depends so much on your own needs. But if a 5D3 is what you need then I hope Canon releases it sooner rather than later for you.
But here's a thought. The RRP of the 5D2 is £2,300 here in the UK so it may well be that for the first few months of availability the 5D3 will be at least that much. Actually, given the depreciation of Sterling since the launch of the 5D2, the 5D3 may actually cost rather more than that. So, rather naughtily I suppose, can I point out that you can go out right now and get an EOS 1D Mark IV (reviews here
) from Park Cameras for, to name but one, for £3,400....