Here's what I'm betting:
The GTX 480 will be about 10%-15% faster than a single 5870 1GB overall. However in extremely tessellation limited situations, AKA the Heaven Demo, it will actually be as fast or slightly faster than the 5970 dual card. However in your average gaming situation it will not be anywhere near the 5970 unless x-fire isn't functioning. Fermi will also have some issues with driver/game support similar to the 8800GTX at launch.
The GTX 470 will be about as fast as the 5870, again faster in some situations or in specific games yet slower in others. In some situations it will be bested by a 5850 at stock or slightly overclocked.
If/when AMD releases the 5890 2gb(I'm betting 2gb frame buffer) it will include a larger tessellation block and some manufacturing tweaks and will be at least 10% faster than the GTX480 in all but very limited situations. Also there will be more availability of the 5970 with some cards being in stock at most major retailers. AMD will shift their marketing to this card...and defend the performance crown until we see a dual Fermi card.
As far as availability goes: the GTX 470 will be available shortly after launch in low but existent quantities...however the GTX 480 will be scarce and they will be at a premium price until June.
Dual GPU Fermi Card: not gonna see it till yields improve and either a complete re-spin is done (B1 revision) or 28nm solves some of the transistor leakage and defect issues.
I've been contemplating starting a thread here titled:
"My GTX 480/470 is overheating" but I think that may be jumping the gun and would be considered trolling despite the humor associated with it.
After all is said and done everyone will realize that Fermi is not a Graphics chip at all, it designed for something different, it's made for GPGPU and has been pulled off the bench to do double duty against AMDTi's truly dedicated graphics chip.
I believe Fermi really is a giant step forward....but that we won't see the tangible benefits to the gaming community until it's next gen.....and/or a die shrink.