I do exactly this, and it works better than the AT&T-branded BlackBerrys. No problems of any kind. Mine came unlocked from Horizon Wireless
, an all-around superior outfit. On the AT&T-branded Bolds you cannot turn off 3G to save battery life, but with all the other carrier-released ones you can.
I have the $30/month BIS add-on, the basic 450 minutes voice plan for $40, and the $20/month unlimited SMS/MMS add-on. Works like a charm. Full internet, all data services like push mail and BBM, BlackBerry App World, you name it.
Plus, if I feel like using another device like a RAZR2 V9x (because it's slim and doesn't bug me with email) or a Nokia (I have a bunch!), I just pop in my SIM card and I have full MEdia Net internet services automatically. Nothing to configure, no need to notify AT&T. When I want my BlackBerry again I pop the SIM card back in and I'm immediately up and running again. Sweet!