Microsoft won't be dropping support for Windows XP anytime soon, and has recently announced that they will actually commit to support the OS into 2014. That will be 13 years after XP was originally released, and while it seems like a long time it is actually only a three-year extension compared to their previous operating systems.
The extended support for XP is undoubtedly driven by the fervor of many businesses and people upset over the cost of upgrading to Vista, for both the OS itself and the more powerful hardware required, along with the numerous deployment issues which still linger in some areas today. They will also still be supporting the "downgrading" of Vista to XP, an option that may cost more for some but may be worth it to them.