I just watched the HDNet Megadeth concert, and I too was brought here via googling Dave Mustaine Lip synch. Amazing for a thread started over two years ago and not posted in for almost a year still tops the google search engines (atleast for DM lip synching).
Wasn't going to post, but I felt that I had to chime in because of the numerous thoughts posted here. So many things to comment on.
First off I have to say that Megadeth garners much respect from me simply from the fact that they did not sell out like their "counterpart" Metallica. I did find it ironic though that Dave was wearing a Kiss t-shirt...who some would say is the biggest sellout band of them all (although I still love Kiss):rockin:.
As mentioned by a couple posters, Mustaine can be a douchbag at times...but I find the members of Metallica to be so more often than not. The comment "divas" was mentioned...it fits!
About 4 songs into the show I was overwhelmingly compelled to google Dave Mustain Lip synching. Reading through the thread, watching the show, I had to bust out laughing to come across a post that admitted ending up here via the same method...then it was followed by quite a few more!
I would like to say that I am still on the fence on this one. I do not want to believe it, but sometimes it really seemed quite obvious. The only thing that keeps me thinking maybe he wasn't is that there were a few times when he spoke to the crowd. When he did this I noticed a couple things:
1. He talks funny to begin with. He doesn't seem to enunciate his words using a pronounced mouth movement as most people do. He could probably be a good ventriloquist with practice. Because of this, it could also make his singing look funny (lip synched)
2. When he talked to the audience, at times he was a bit farther from the mic than you typically see. Some people talked of this tell, which usually IS a give away, but I have to say this mic seemed to pick up his voice pretty well from 6-12 inches. I in no way believe the spoken parts were lip-synched, so thus I have to believe the mic could pick up his voice while singing 6-12 inches away.
As of today, it seems this debate still lives, with this thread being the #1 source of info on the subject.

Perhaps someday we may finally get an a definitive answer...it was live with a minorly badly synched audio and video feed, it was lip-synched, there was post production overdub, or just Dave with funny mouth movements. We may never know.
Also wanted to state my opinion that Megadeth and Metallica were never really Thrash/Speed-Metal/Death Metal bands (Slayer could probably fall there, with Anthrax being borderline). While they did have songs that crossed into that area, I do not think that they could really be classified completely in that category. And to be classified as a Thrash/Speed-Metal/Death Metal band, I think the majority of your songs have to fall into that category. This of course is a very gray area and is really a matter of opinion, so it can be debated till the end of time.