Exactly the same for me. It worked fine for the first day (and confirmed as validated, showing the "Windows Genuine Advantage" logo in my computer properties), then when I switched back on next day it said I wasn't activated!
I guess I now have 30 days before it locks up, even though I have a valid RC1 key and this is a fresh install from a formatted hard drive.
This is the second time it's happened too. The first time was after an upgrade of Windows XP Pro SP2, which worked until I switched off as well. I then did a fresh install and it still does it.
Not very impressed.