I can assign all new users to a certain role group easily enough, but I'd like them then to be able to down-select their new role group on their own (with a few groups excluded as an option.)  

My site requires lots of different role groups, each one is limited to post editing for a unique category.  It basically needs to be a step in the registration process before the user can begin to use my site. It would be a pain (and very slow) to have to moderate all of those requests by myself.

To try and make it clearer … here's how I'd like it to go:

1) A new user, Johnny, signs up and is automatically assigned to the role group "New User."

2) "New User" role group allows Johnny to select which category he belongs to.  He must be restricted from selecting role groups higher than his — and in fact, can only select a role group lower than the one he is currently in, which is "New User."

3) Johnny selects "Group B."  Now, he is assigned to "Group B," which doesn't allow him to change role group anymore.  

Is there a way to configure the "New User" role group to allow this functionality?

Aha! Figured it out!

Use Capability Manager to create new roles in WP.

Use WP Roles at Registration to allow user to select their role when they sign up.

Use Role Scoper to assign particular roles to specific groups to control their editing by post category!

Hooooraaaay!!  Now I just need to figure out if i can make this work somehow with facebook connect.

Thanks for letting me talk it out here!  I hope this is useful for someone in the future.