is there a shortcut key to do this...as there is in Google Reader (the M key)?

But you have to be in a post to do this. I accidentally marked all posts as read, and wish there was a way to undo that, without having to traipse through each post. I am looking at migrating over from Feedreader, so that I am not tied to my home computer.

it would be much easier if the button showed up in the list view. thanks.

I finally found it - didn't realize you have to "open" the topic to see the button. Perhaps put that in your "how to" section, then?