I learned on another forum that Mike Nelson, the head writer of Mystery Science Theater 3000 fame, has started a site called "Rifftrax." He basically does the MST3K thing, and puts it to MP3. You buy the MP3 for $1.99, rent the movie it goes to, synch your MP3 player to the movie (instructions on the MP3, and are really easy) and start laughing. I just did it for the movie "Roadhouse". Not the exact experience as MST3K, as there's just Mike, but I was cracking up.
He just started and only has Rifftrax out for "Roadhouse" and "The Fifth Element", but I've already put "The Fifth Element" in my Netflix queue.
http://www.rifftrax.com