The IE team announced what the user agent string for IE 8 will be:
Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0)
This particular one is for Vista. They’ve decided not to use the ‘b’ in ‘MSIE 8.0b’ for the beta version this time due to issues they’ve encountered in the past with that approach. I’ve updated my User Agent Switcher import file to contain the string for Internet Explorer 8 on Vista and XP. As always you can get to that file from the right menu under the “My Testing Tools” section: User-Agent Information and Tools. Load up the new file and go see if your browser detection script will work with IE’s new string.
I also updated it with FireFox 2.0.12 for Vista.