Which device and what "data capabilities"?
iPhone 3GS.
Data capabilities = data transfer, you know, internet, email... everything but voice on the 3G network. Wifi was fine.
But the day after I started this thread, I simply restored from a previous backup, installed iOS4.0,
declined the carrier update and rebooted. Going into "Settings" updated the carrier information automagically (has prescribed on Mac forums) and everything worked fine.
Since then, I've jailbroken with
Welcome page, unlocked with ultrasn0w and everything is great!
(I didn't think this thread was still active.)