I'm a Cingular customer, and discovered that Safari really doesn't work for setting up a BIS account, but there's a reason I keep a copy of FireFox installed on my Mac..
Fired that up, and got the BIS account working without issue. Once the account was setup, I've (thus far) been able to do every thing else I've needed to with Safari. There was a time, not too long ago, when FireFox wouldn't even work -- I was forced into IE, and THAT made me shudder...
(And, off topic, after using FireFox since the beginning of time, Netscape before it, and Mosaic even before Netscape, I wholeheartedly endorse Safari