It took about 26 hours...
...for an activation email to appear (early Tuesday morning to 3 am today) in my case.
I suppose it could go either way, but the email I used for pre-registration worked immediately I had downloaded the app.
I find BBM pretty good, and on Android at least I can see that the incoming message isn't marked as read until I've touched the screen for the chat in question so it really can only be the fault of the reader when a message isn't heeded.
If only BB had launched the Z10/Q10 with BBOS 10.2 then it might have sold better, it's actually a very nice phone but in March it was still incomplete from an OS point of view.