Hopefully you'll hear plenty about this new Cross Site Script library from you local developer security events!!
The AntiXSS Library is a white-list (encodes all characters not known to be safe) encoder for html and urls and it significantly improves on the encoder provided with "Server.HtmlEncode" and "Server.UrlEncode".
The first release is for Framework 2.0 but support for Frameworks 1.0 and 1.1 to follow shortly.
You can download from http://www.microsoft.com/downloads/details.aspx?FamilyID=9A2B9C92-7AD9-496C-9A89-AF08DE2E5982&displaylang=en