the first beta of IE7 has been released for MSDN subscribers.
So far I haven't noticed any problems, will keep you posted if I do<

I hope the CSS problems have been fixed.<

not completely :), there is quite a big list of fixes for beta2 already<

you can read what they are up to here http://blogs.msdn.com/ie/archive/2005/07/29/445242.aspx<

There are I believe some unoffical beta 2 builds out as well too if you hunt around a little, the core team started doing beta 2 builds a couple of weeks ago I believe. Theres still quite a lot missing from the beta 1 build though, some of the improved CSS validation is in there and you'll notice some sites that previously looked fine now don't look quite as good becuase of it.<

Was IE 7 supposed to be for Windows XP / Windows Vista only?<

To the best of my knowledge yes. It was supposed to be for WinXP SP2 and Win Vista.
I seem to remember reading somewhere that it had to do with the security features added to SP2 and on.<