As you probably know: to use a different SIM, you will need to get a SIM unlock code, which may be hard if you are not a customer (but some unlock services can help with this). Once you do this, you can get cheap data and calls abroad, (much cheaper for calls than the discontinued unlimited data plan, and comparable for data). Without unlocking, you are restricted to the supplied Vodafone SIM, which is very very expensive abroad without the grandfathered plan.
In the US, there is another problem to do with the carrier lock. Without doing more, you cannot use GSM in the US, the phone will detect the signal but will not allow a connection to be made. Team Black Hat offer a fix for this (for $100) that allow the phone to work on GSM in the US. Lots of caveats, including that you cannot get any more system upgrades, as these will break the hack.
Lastly, the Droid 2 Global cannot receive the T-Mobile 3G band at all (no radio for this). So on T-mobile, even with the $100 hack, you are restricted to EDGE/GPRS, an expensive solution for that. AT&T 3G will work after the hack.
Hope that helps.