I am excited as well in AirPlay Mirroring so i watched several times in the demo. It seems video plays with Quicktime. So lot's of video formats are not compatible like mkv's, iso's etc...
If somehow i am able to mirror VLC i will run and buy Apple TV at once.
From what I can see it's just screen sharing isn't it? Play something in VLC in fullscreen and you're fine. They use Quicktime for the demo because it's an Apple product but I don't believe it's a limitation.
If Apple have to throw Quicktime under the bus to sell more Apple TVs and Macs I think they'll happily do it.
What you are saying is true, however it is a rather messy solution. Assuming that the next AppleTV will support 1080p output you could not just mirror your screen to get the best quality. The source film is in 1080p, gets scaled down to screen res. which is transmitted to the AppleTV and then gets scaled up to 1080p again - resulting in a easily visible quality loss.
I hope they treat the screen as a "second screen" which is mirrored...
Looking at Apple's page, it seems like it doesn't only mirror output of apps such as Quicktime, but is able to do desktop mirroring, so you can play videos in VLC and mirror the entire screen.
If somehow i am able to mirror VLC i will run and buy Apple TV at once.