One would logically expect nVidia to jump into the PCI Express boat quickly because of how quickly they are embracing the technology with their video cards. I have no idea on the former though.
a) Paper launch before the summer holiday, the first few, expensive, chips practically available for christmas
b) If you're thinking of "consumer MBs with PCIe slots based on nForce available for purchase, at reasonable prices", I guess fall.
Good point from Mad_Pyro, though.