With it taking a 4870X2 or GTX295 just to start showing the age of even an 8x lane PCIe slot, I don't think we are needing PCIe 3.0 too soon.
16x lanes of PCIe 2.0 is more than enough for the future. Although it might pose a problem for those wanting to run Crossfire or SLI on P55 boards when the GTX3xx and HD5xxx range come out. I guess we will have to see just how much the high end cards from each camp can output.