Can Apple TV AirPlay mirroring use a "secondary" screen/display? Can Apple TV enable me to treat my TV as a wireless secondary screen (as one can do when directly connecting the TV via a cable), rather than simply mirroring my primary screen?  I'd love to stream content to the TV while still using my laptop to do work.

(I know you can stream iTunes to the TV but that's very limited in what it can stream/play, e.g. can't stream content from arbitrary video websites.)
iPad 2 + Air Display. http://avatron.com/apps/air-display

Use Air Display to turn your iPad into a second monitor (totally cool in itself!), and then mirror your iPad to your apple tv. 

OR, if you, like me, only have an iPad 1, then you can use your iPad as the main monitor and mirror your laptop screen. 

Either way, you end up with three screens going, two of them the same.

That's the only way I know. Seems crazy that Apple can't come out with an update that allows the apple tv to be a second, rather than mirrored, screen.