I would love to see a hack where each registered user is given their own individual forum, viewable in their profile. In essence, using vB to power a guestbook for their profile.
thoughts?

IIRC wluke's karma hack will allow people to leave comments about other users, these comments will then be displayed in the users profile. Is this close to what you want?

This is close. I actually hacked 115 to do it but never had all the issues taken care of.
I'm not necessarily looking for people to leave comments about the person. I think of it more as people leaving questions/polls in their profile for other users to respond to. For example, if you go to http://bolt.com, they have a feature called the tagbook that does just this (tyr clicking on a username and then click tagbook). I think the more you can enhance the user profile, the more sticky your site is. It truly develops community and users' personality within that community.
Will wluke's hack allow users to edit/moderate their own guestbook?
I don't want a feature that causes tension between my users, but rather creates dialog amongst them.

No. It was never intended to be that complex.

what you want appears to be a visible pm system. I am not sure if it would add any more than one you can currently do either in a chit chat forum or through pm but creating a visible pm system wouldn't be that hard, just copy all the pm code, set it to visisible to anyone and prevent other users from deleting them. Quite a major hack could be done on the back of something already there.