If anyone's still watching this thread... I finally figured it out, it was My Own Darn Fault™. :)
The problem was that I installed a system utility called BetterTouchTool - it lets you do all kinds of custom configuring for input devices (I actually installed it to inhibit Magic Mouse scrolling when the mouse button is down, because the scrolling drives me nuts when I'm trying to move things in the arrange or piano roll windows). I ended up calling Apple a couple of times, and though they didn't exactly figure it out (not that they should have; they didn't know about the extension), they were very helpful and patient, and didn't charge me.
Anyhow, I believe the root of the issue was that BetterTouchTool was confusing/overriding my system Accessibility prefs, making it think that I wanted the computer to be able to speak to me. As a result, it forced the plugs to stay in controls view so that all controls would have text that it could identity and speak (fancy plug-in GUI's are just a graphic with words imbedded in the artwork, so they're not decipherable if you wanted the computer to speak to you).
Once I disabled BetterTouchTool, plug-in views went back to normal (and oddly, I could re-enable it, and everything worked after that). What was very helpful in diagnosing it was that Apple support had me try Logic with a test user (System Preferences\Users and Groups\ create a test user, switch to this user, etc.) and it worked fine, because BetterTouchTool wasn't enabled for the test user.
