For some time now, NVIDIA has maintained that Intel's high licensing fees have kept the nForce away from the Pentium 4. Given the fact Intel already has licensing agreements with ATI, SiS, and VIA, it seems unlikely licensing fees would be lowered just to accommodate NVIDIA. Remember, Intel has a healthy chipset business of its own to protect, too. So if the rumors are true, what made NVIDIA bite the bullet?
With AMD betting the farm on the Athlon 64, NVIDIA may have picked up a Pentium 4 bus license as an insurance policy against disaster. The fact there is a massive market for Pentium 4 chipsets is also a pretty good reason to suck up Intel's licensing fees. However, I'm not sure how well an nForce chipset for the Pentium 4 would go over with AMD, whose relationship with NVIDIA appears to have grown much closer over the last few months.