What devices can I Airplay to?

Yes.

Airplay is really a technology platform that allows for streaming media on a local network. It isn't concerned with the type of device that is producing the media or consuming it.

To start with, support may be limited and may not come from Apple. However, certainly that will change with demand. For example:

• There is an iOS app called AirPlayer which allows streaming of media to an iPhone/iPod/iPad.
• There is a Mac proof-of-concept app also called AirPlayer which allows streaming of media to a Mac.

Looks like Banana TV is a currently shipping Mac app (not just a proof-of-concept) that will play Airplay video or images from your iPhone/iPad.

(TUAW review)

Airserver allows you to stream Airplay audio and video from your iOS device to your Mac. Works great.