I noticed when I was using SO and meta today that I was hoping back and forth between the two to see if I had any notifications on the other. It would be cool/nice to see notifications pop-ups for other sites that you are registered under the same Open-ID. The same could be said about the letter icon to the left of your name. Maybe you could have multiple for each site your registered to, or turn red if something happens somewhere else as well.
Just some more food for thought. Anyone else have any ideas for something like this?