Question Locked, unlocked, Cingular (US)

I now have a unlocked PDA2k and Cingular SIM. I am considering a Cingular 8525 since the price is so much better than unlocked devices. Can someone clarify the exact difference between unlocked and locked - it is just what SIMs will work right. If I go with the 8525 can I install another ROM version, maybe the worldwide, non Cingular specific ROM? Can someone comment on the major differences between the i-mate and Cingular ROM?

Thanks much!