A colleague of mine had a Blackberry with Cingular which then transitioned to AT&T. They didn't have the capability to just assign a device a data plan without a phone number like he wanted.

However, the first-level CSR set up the BB with a phone number along with the normal $40 BB data plan. They simply just didn't tie a voice plan to the device which meant that if he ever made a call out from the BB, he would be billed $.50/minute or some other outrageous amount.

He used the BB for years like that without an issue.

You may also look into the DataJack which runs on T-Mobile's network. I presume there's a SIM inside the dongle that's ripe for the plucking. Unfortunately, there would be no voice service but you could just use Google Voice on an Android device. The DataJack seems very new, so you might be blazing new trails.