Now days pretty much yes, before the Cingular/ATT merger I think Cingular was the only carrier the offered BlacBerry's unlocked out of the box which has since ended. Correct me if I'm wrong but the 7230 is a T-Mobile device whom lock all their BlackBerry to the T-Mobile network. If you have a SIM from another carrier pop it in, if you get and "invalid SIM" message chances are its not unlocked.
Number portability has nothing to do with locking a phone.
Phones (and BBs) are subsidized. So the carriers lock them so that you have to use them on their network, at least until they get the subsidy back (in phone charges).
WNP allows you to move a number from one carrier to another, and nothing to do with unlocking phones.