Internet Explorer 8 sends an updated user-agent string when interacting with web servers, and with the release of Windows 7 Beta 1. Each of these events has a small impact on the User-Agent string.

The Trident/4.0 User-Agent String: In order to help users visit sites that block the “MSIE 8.0” user-agent string, IE8 will send the “MSIE 7.0” version information when viewing sites with Compatibility View enabled. As Scott described last August, a new “Trident” token in the User-Agent string allows your code to detect Internet Explorer 8 clients even when they are using the Compatibility View feature.

Full Article