You can download all the crap i want for your BB. It is not locked. It will only play MIDI ring tones and there are a few other minor annoyances, but you can add apps.
One way you hear "locked" in in regard to a carrier lock. For instance Cingular Blackberries may be locked to Cingular. You can have them unlocked to work on T-Mobile. I suppose a 7100i could be locked to Nextel (or Boost or Telus or Southern Linc or something), but unlocking it would only get you other iDEN ability to the carrier with whom you signed.