Petz, I'm 99% sure that Intel variant of 64 bit P4 will not be AMD64 compatible. Why should they? They can afford not to be.
Microsoft. It all comes down to them. If Microsoft had agreed behind closed doors long ago to support another instruction set, then that is what Intel would have done. Rumors say Microsoft told Intel they wouldn't support an additional instruction set. Sure Intel can whip up Linux support very easily, but MSFT still holds a lot of cards. Does anyone think a new instruction set would fly without MSFT support? We may all hate MSFT, but the reality for now is they have the biggest OS market share.
Intel also probably believes they can outdo AMD on their x86-64 implementation in the long run, and over time (5 years down the road), people will probably believe x86-64 was an Intel creation if Intel manages to stay ahead in market share and performance. They can also play off the momentum x86-64 has already built (OS, drivers, etc.), and have a product to market sooner.