Before resorting to a full wipe and reinstall, I decided to get a bit smarter about this and use the info readily available to me (after all, I've been developing software for more than half my life...should have been my first approach!).

I have QuickLaunch installed, and found that you can specify the Blackberry EventLog as an application for it to launch, making it VERY easy to try an action, then quickly switch to the eventlog to see what the Blackberry OS thought of this.

So:

I brought up the event log via quicklaunch and wiped the log. Then I attempted to edit the active profile (click profile icon, select advanced, click on active profile) -- and then immediately took another look at the event log. Lo and behold, the logged exception showed that there was a missing Facebook resource!

I checked options/advanced/applications, and found that both versions of Facebook were somehow installed on my 8900 at the same time -- both 1.5 and 1.6. I deleted 1.5 and rebooted the 8900. Still no joy. So -- I deleted 1.6 and rebooted....and...Success! I can now edit my profiles again.

On the theory that the combo of 1.5 and 1.6 somehow indicated a compromised install before, after making sure that there were no traces of 1.5 left on my 8900, I used App World to reinstall 1.6, and happily enough, I can still edit profiles properly.

Best guess now is that version 1.5 was still partially installed, and was still hooking the OS, indicating that there should be a link between the current profile and various Facebook resources (since profile options can respond to FB events as well as other events). But...the fact that 1.5 was only a partial install meant that a few files were missing, in my case these included the resource files needed to link FB to the OS's profile manager.

Suggestion for anyone who ever runs into weird behavior that seems inexplicable: use the EventLog -- it's an incredibly quick way to get some clues regarding which application is failing even when there are no visible signs in the user interface of your blackberry. Next best thing to a debugger if your not a developer...