Let's not confuse domestic roaming with international roaming. If you take a North American (I was going to say US, but don't want to offend anyone)
GSM/GPRS BlackBerry, it will stick to the carrier used by the SIM card and not roam around. The carriers normally do not allow you to roam in your home country. The example of the Virgin Islands is this: Cingular has an 850 MHz network there. T-Mobile has no network there. If you take a 7290 or 7100t from T-Mobile to the USVI, it will work - on Cingular - without switching the SIM card. International roaming with a GSM/GPRS BB is easy. The domestic carriers have tons of international roaming agreements, where GSM is MUCH better than in North America. So you travel and roam. Both voice and data.