First: My desktop runs Win 7 7600 X64 and runs it great. I have but one issue with it and that is that I had to turn off Hybrid sleep or about 30 seconds after it comes out of sleep it hangs and I must do a hard shutdown. This seems to be a cross platform issue and I think it is the chipset drivers and will be solved slowly part and parcel by the individual vendors updating thier drivers.
I have eight computers running in my home; 6 @ XP Pro, 1 @ Vista, and Mine at Win 7 Ultimate. I have no intentions of upgrading the others to 7 untill it becomes necessary to do so. Why should I? They run great and havn't ceased handling anything they are being used for. All are current builds, dual core 3+Ghz with 2Gig of ram each. They are capable of 7 but 6 @ $200 is big bucks.
I work for the Post Office but repair and upgrade computers on the side for little more than the fun of it. I'm constantly asked when I recieve a computer for repair or upgrade (with nervousness in their voice) "do I need to upgrade to (then) Vista and (Now) Win 7? I ask them what they ues thier computer for and make a recomendation. Almost exclusively I recommend they stick with XP untill they make the upgrade to a whole new computer. Then I recommend they buy a name brand at a reasonable price.
Most people that I meet use thier computer for surfing the internet, paying bills, email, chat, photo archiving... Nothing that requires Quad core 8Gig of ram Win 7... I think Vista got a bad rap in the begining but with the service packs it became a reasonable OS. I do see that people would switch from Vista to 7 even though Vista is running stable for them now because many of these are people that switched from XP to Vista.
XP will go away and like Qdos said it will happen mostly through upgrading whole systms. For these XP users why should they spend $150.00 for a new OS when they can get the whole system for under $300.00. Lets get real here.