May not be everyone's cup of tea, but the new Windows 7 Beta is being released on the 9th Jan 2009 afternoon (By this I assume it will only be available at Western US noon time). Apparently it's only valid for the first 2.5 million downloaders and the beta is valid till 1st Aug 2009.
From the Microsoft website -
Over the past few years, you've asked us to make some changes to Windows. We listened closely. Now it's time to share an early look at how we've used your feedback. Windows 7 is faster, more reliable, and makes it easier to do what you want. Both the everyday things and the killer "is that really possible?" things. Dig into this site to see what's coming. Want to try the Beta? Come back the afternoon of January 9. And rememberthis is just a preview. There's more to come.