If people start PMing in the new system, it might cause problems with transferring old messages, which is something we hoped to be able to do for everyone. That's the reason we didn't enable it today.
Ian suggested just enabling the new PM system and letting people access the old system for a while to get messages they may need from there. I think that's a good idea, but I want to discuss it with the rest of the BOD before going ahead with it.