Ryan et al.,
You will never believe what the problem was. I am so pitifully grateful to figure this out that I can't even think straight.
It was Norton *(O&&^%^&%*( Antivirus. My firewall was turned off when I tested with no luck. BUT NORTON antivirus 2006 inserts &*^(&*^ INTERNET WORM PROTECTION that treats every attempt at sniffing the LAN as though it were some sort of attempt an nefarious wrongdoing.
The effect is that THOUGH THE MACHINE IS STILL BROWSABLE (sometimes), and all the right ports are open, and so on and so on, Norton takes SO LONG TO VERIFY that it's OK to access the machine that it times out most of the time. I just tested this, and WITH the worm protection on, SOME machines on my LAN see the machine running WMC RIGHT AWAY, and others see it only after about a 1-5 minute delay. The ROKU appears to be one of those machines that Norton is paranoid about. God only knows why. Anyway, when Norton is ON, but WORM PROTECTION is turned off (important distinction, as I am sure NO ONE is going to turn Antivirus completely off!), it works fine. It doesn't help to leave worm protection on but allow exceptions. The exceptions don't seem to work right in Norton AV 2006. If you have an earlier version of Norton, it's fine... it's just NAV 2006 that gives it fits, because it's HALF firewall, half AV and not very good at either.
That explains, at least in my case, why the 'no libraries found' behavior is so erratic, works sometimes, stops working, etc., etc., and is absolutely irrelevant to whether WMC is reinstalled, uninstalled, whether the firewall is on or off, etc etc., and why for SOME people, it works just fine and for other folks it's a ^(&^(*&^( headache.
And Norton AV is the ONE THING I updated at about the time that the Roku stopped working.
Geezus H. Jehominy and his fat brother harry, I can't believe THAT was the problem... But it was.
Not you Roku guys' fault, and I apologize for causing a stink with the exec team. Though it WAS nice to meet them by email <sheepish grin>.