XSS: Welcome to 2003 (or thereabouts) [ajaxian.com]
Ajaxian mentions SafeErb for Rails, an add-on to help secure that user input is safe. It does so by checking if you explicitly call a certain method to escape the user content.
OpenACS, the base for .LRN, has been doing this for awile now. We took a different approach. All content is escaped by default, and the programmer must decide when to let through unescaped content. Either way it's something all Web frameworks should support if they allow users to enter HTML.
You may request notification for Solution Grove Blog.