When we were at MGM back in May, they were holding the late-night EMH test. One night at MGM, a camera crew began setting up near the Brown Derby around dusk. We were on our way to Fantasmic, so we didn't give it more than a casual glance.
A couple hours later, we were still wandering the park and it looked like the crew had just finished filming. We walked past the camera setup, and saw several other guests getting autographs from a guy. I could be wrong, but I think it was Greg Proops (sp?) who was on ABC's "Who's Line..." show.
The sorcerer's hat was all lit up can clearly being used as a backdrop. My guess was that they shot some sort of WDW or ABC promotional footage that night.
But, to add to what WDSearcher said, this taping appeared to be no different than any other Disney events you might randomly stumble across at any of the parks. I remember taking one of the MGM tours back in '97 and getting a look at some of the sets from the HBO series "From the Earth to the Moon." But it's now been a while since MGM was used as a working studio.