So here is a kludge workaround to mark all feeds as read if you use IE7 as your RSS Reader:

In your Feeds section of your Favorites Center in IE7, right click and create a folder(s) in which to put all of your feeds. Highlight the folder with your feeds and click on the arrow that appears to the far right in that feeds window. That will open up all your feeds in separate tabs, and in the process, mark all of them read (assuming you have that preference turned on — if you don’t, right click in the feeds section and click Proprties. Then click on Automatically Mark Feed As Read When Reading a Feed).

Yes, this is ugly. But it is faster than going through all of your 50s or 100s of feeds one by one. Just make sure  you don’t break IE7 by opening up too many tabs

By the way, I now use Dare’s RSS Bandit.