Vista fans say there's no major reason to wait, but remember one thing. If you downgrade to Vista now, you will be working around your operating system, while in reality, it should be your operating system working for you. There is absolutely nothing you gain by downgrading besides some eye candy which looks sleek at first, but its not a big deal when you get used to it.
I can see making the switch if you are an uber gamer when games such as UT2007 and Crysis emerge, but ONLY if you have a DX10 capable card.