I applied the .Net Service Pack 1 to a Windows 2003 server last night and the web application served from it stopped submitting postbacks. When the login page was sent to the client (IE 6 on XP, 98, and 2003 that's all I had to test with at the time), the javascript was preventing the postback from the submit button. The validators all worked, but the page just would not post back. Click the button...nothing. The development machine with XP Pro and the Service Pack 1 did not have the same problems. I ended up rolling back the patch to get things working as it was late and I didn't find any solutions right away, but this morining I found some other possible workarounds.
There is a discussion of the problem and several potential solutions on the Channel 9 site.