I think that we should not limit the length, but not allow "_"s. Since spaces can be used there isn't a need for "_"s, and if spaces are used instead the name will line wrap.
Which brings up issues of /msg'ing and other bits of unpleasantess. I don't see the need for usernames that long. Others have made good points, and I think a longer limit would probably be better; 20 would most likely be acceptable.