Hi andy, I discussed this later on my blog also:
http://www.rahulsingla.com/blog/2010/...

I earlier went into details about this, and found the entire architecture built around this. For a clean installation, you can overcome this by using email id as the username as suggested above. However, I don't see any reasonable way to clean this up across 50 portals. However, I could not understand when you said:

 Not surprising at all that you would get a bunch of people choosing the same golf-related usernames.  Many of them used the same username and password

Did you mean you have different people accidently choosing same username/password across portals, and therefore different people are now using the same username to access different portals on your installation. If that's so, this is really messy. The interesting part would have been that updating First Name/Last Name on a prtal should have updated across portals. So, there must be a race going on between people sharing the usernames to get their First Name/Last Name right, and they might be confused how their First Name gets upated every now and then??