I think it would make more sense to just leave unread messages blank and put an icon beside the ones you've already read and of course the ones that were updated since you last replied. This would be great in the case that you replied to a thread a couple of days or so ago and missed its bump to the front page. It would not appear on your recent activity page because it was bumped off of page 1 of that by new posts you made since then. 80% of my activity here is just reading so right now I see a lot of "unread" icons.
Also, love the fact that the nav bar doesn't follow you around anymore. Now I can type on my iPhone without moving it out of the way first! And forum ratings were much needed!
The only thing I don't like so far is the inability to close the rating tab on mobile Safari. If I tap the pop bumper button a second time it doesn't close. Though I realize the button was designed to open when mousing over it, which is obviously impossible on iOS. Just a suggestion.
Edit: Oops! Tapping outside of the button works. But maybe consider that too to avoid confusion.