They're remarkably easy to sign up for if you haven't already, and once you have one, you can start adding games. For non-WoW games, you'll need your CD keys (I had my D2 keys handly, but alas, I can't find my Starcraft or Warcraft III CD keys, which sort of bums me out). For WoW games, you'll just need your old account number and password. When your WoW account is in, you'll start signing in with the Email and password you use for your Battle.net account.
What's the advantage of signing up for these new accounts? To start, you get easy access direct download for all games you have tied to the account. Also, if you have multiple WoW accounts tied to one battle.net login, you can sign in with your battle.net account and choose which WoW account you want to access from a list, which, at the least, means fewer passwords to remember. Finally, you won't have to be sad like me if you can't find your CD keys: They'll be automatically tied into the account.
There's more features to come, of course. Zarhym specifically mentions the possibility of crossover communication between Blizzard games, and there's always the possibility of other functionality as well. I'm all for account-wide achievements, myself. A special WoW pet for defeating Diablo 3 on Hell mode? Yes please!