Ortsio told me that if you have existing SMF users, when they login through the bridge they will be automatically registered as Mambo users and they won't even notice.
Note that there are problems with SMF profile altering with the bridge at the moment, if your users decide to change their username or p/w in SMF, this will not be reflected in Mambo. I'm going to use the bridge as a way to run SMF in Mambo and leave the Mambo registration out for the moment until the bridge handles profiling properly as well.
That said, it's an excellent way to get a top quality forum into a Mambo site

Cheers,
Damian