I don’t know if you’ve been spending much time on stackoverflow lately, but if you are you’ll know it’s a kind of addictive experience. There’s all types of developer questions on there. The tagging structure is neat and allows you to easily keep track of certain topics.
There’s a growing list of VSTO related questions there, so along with the MSDN VSTO forums you are pretty likely to have any VSTO related question answered. stackoverflow doesn’t have any RSS feeds of their tags at this stage, but I hear it is high on the wishlist.
There’s also an RSS feed of the stackoverflow VSTO questions (although it is easy to miss, being right down the bottom of the page).
[Plus, if you subscribe to the MSDN VSTO forum feed, you’ll be happy to know that the feed format has been fixed. A little while back I was complaining about how the feed was a mess with html tags all mixed in. That was fixed about a month ago. Yay!]