It's a tricky thing, choosing an online handle. You've gotta stick with it forever, so the dumb inside joke or ironically stupid insult you choose will be your name forevermore.
The Week In Games: Fishing For Mysteries And Gang Wars
As a general rule, Blizzard gives users a one-time-only chance to change their mind on their "BattleTag." But, given that a good number of people who played the beta of Diablo III may have used that chance already, they've extended the privilige to all their users a second time. So, if you're still not happy with your battletag, you can hop on into your account settings page and change it again.
Man, I wonder how many people have tried to change their Battletag to "Error" in the hopes of landing Error#3737.
Battletag: Free Name Change Update [Blizzard forums]