Just my 2copper. 5 WOWs, Starcraft 1, Diablo II and you are pretty much full (only 8 accounts per B.Net). With SC2 and Diablo III coming, it seems like confusing to put too many things on one account, so I got 5 B.Net accounts. That way if I box Diablo III (if you can), then I have room to grow. If I want to have a second SC2 (not sure why) account, I can.

Other things mentioned. Game pets (from promotions) are B.Net account wide, so one B.Net account would be good, but bought pets are only WOW wide, so you would need 5 bought pets, no matter what your B.Net account situation is. Authenticators only work for 1 B.Net account login at the moment. You can use the same physical authenticators on multiple WOWs, but you have to push the generate button 5 times. The upside, is you can use the same number, (saying again, same number) on seperate B.Net logins (if you have 5 B.Net accounts I mean). So 5 B.Net accounts lets you broadcast the number, buf if you use 1 B.Net account, you have to push the button (on authenticator) 5 times, which takes a while and is annoying.

I personally want a authenticator or system that detects which computer I am using, not make me type in a number. That way if some other person half way around the world tries to use my login, guess what, you can't, since it isn't from my computer.