We've put up some more documentation on MSDN regarding User Agent string and posted on the IE team blog at http://blogs.msdn.com/ie/archive/2005/09/01/459541.aspx 

At the same time Sandi Hardmeier posted about this issue at http://www.ie-vista.com/sites.html as she has been blocked from accessing some sites while beta testing IE7. Having the browser pretend to be IE6 by modifying the string solved the problem.

Fortunately the majority of sites are not affected by this. However there are still a significant number of high profile sites that block access to content based on the name and version of the browser that is requesting content. This is simply the wrong thing to do. It is certainly great to encourage users to upgrade to a supported version of a browser. However the User Agent string is a notoriously unreliable way to detect the capabilities of the browser as a user may have adjusted their settings and extensions sometimes modify the user agent string themselves. As the user agent string can easily be modified using the registry there is actually no guarantee that the browser is the version that it says it is.
If you run a website please check you are not unnecessarily blocking access based on the browser and version being reported.

Thanks
-Dave

Edited to correct my poor spelling and grammar based on Spellbound's feedback.