I have put Windows 7 RC1 on my X300 (in a new partition). All seems to work pretty well, except that I can't boot into the rescue partition via the blue button - but, as the system was second hand, I don't know if it worked before I installed it.
There are some beta drivers from Lenovo (see this link) but most of my stuff worked with the native Win 7 drivers.
I have a suspicion that installing a new OS can affect the boot manager and make the rescue partition unavailable as I had the same issue with a Lenovo 3000 after installing Linux (again, in a new partition). I didn't ever get round to looking in to that problem.
Has anyone else any thoughts on this?
Long time ThinkPad user: 770, 600, T40 (still in use) now X300 128gb SSD, 4 GB ram (love it! :-)) Windows 7 (still getting around to Linux dual boot)