I think this would be a good idea. When you're someone like me (with few posts and not much reputation), checking for new rep once every visit is overkill, but by the same token, they're rare enough for us less-verbose members of the forum that I wouldn't want to miss one. A popup would be an easy compromise. On top of that, it would give me five more seconds each day to make valuable contributions to the forums during the time I would be checking for new rep!
