He was either lip-synching or singing over pre-recorded vocals. It was pretty disappointing. I'm glad I only paid $15 for my ticket rather than the several hundreds of dollars they were being sold for on eBay.
Was anyone else at the show? Comments? That little blunder ruined the entire thing for me - unless there is some other explanation someone can offer.
Thanks to daymented for digging a little deeper on the purported lip-synching and informing me that Muse was probably using a Vocoder. It was also confirmed that the band did indeed sing, but probably was singing over vocals provided by the Vocoder - at least on one song.
Not nearly as sinister as I first thought, but I'm still a bit turned off by the whole thing. However, I'm sure Muse will survive.