Well, I only talked with one rep from Cingular today, but he confirmed that they have separate data plans for using your phone as a modem, and they start at $20 for 5MB or so, up to $80 for unlimited. That was enough to convince me that my issue was because I was purposely being blocked, and I don't really want to incurr extra charges.
Oh well, I dreamed it was possible once... now I have to figure out what else I'll use my BT dongle for my laptop for.