We have feeds for all the public forums @ www.asp.net/forums now. For example, you can grab the announcements feed here.
Just look for the familiar 
at the bottom of the page when viewing the threads in a forum.
You can filter the list of threads returned by a forumid parameter or a count parameter on the querystring of the rss.aspx page. For example:
http://www.asp.net/Forums/rss.aspx?forumid=11
Returns most recent 25 threads in the Announcements forum
http://www.asp.net/Forums/rss.aspx?forumid=11&count=50
Retuns the most recent 50 threads in the Announcements forum
