RIM writes the OS code, carriers test to their specs and deploy on their own schedule. FWIW Robbers is usually one of the last out of the gate when it comes to OS releases. If you have a GSM BB you can use another GSM carriers 4.2 OS release. A quick search will yield a sticky on the topic.
I would imagine Rogers will only offer the OS on new units. I upgraded my 8700 to the OS 4.2 posted here in the forums and everything works fine. In my case we have our own BES, and being a system admin, I was able to reset my BES activation myself. So far I do like it for the most part. There does not seem to be much of an advantage of having 4.2 vs the 4.1 though.
I upgraded to 4.2 on my 8700R last night. Only issue I have found is the Ringer has the same audio level for MEDIUM and HIGH. Low, and Medium work, but High get's no better then Mediuml.