The next CTP for Katmai has been released. This is the July 2008 CTP. You can download it here.
Here are a few links:
As much as I don't like the idea of QA being handed down to the users, I do think it's important to give feedback if you can. I have seen Microsoft respond to feedback over the years and we've definitely influenced the product, so if you find issues, go ahead and report them.
One incredible annoying thing I want to give feedback on is things like the ReadMe (which points to the installation requirements) and other documents are "downloads" on Connect. Very, very poor UI work.
Also, it doesn't work in Firefox. Guess that's why IE is on my machine :)