Yes, it is completely different. It only speaks Canadian.
Sorry, that was sarcasm.
All of the BlackBerry Handheld OS are written by RIM. The carriers test and release them when they feel like it. There are 8800 versions released by Cingular and who knows else.
You can upgrade to any version that you want. There must be, conservatively, 1000 threads on upgrading your BB, including an excellent Sticky FAQ thread that is very good reading, at the top of the General BlackBerry Discussion forum.