What devices can I Airplay to? With AirPlay will I be able to stream to iPhone, iPod Touch, and/or iTunes, or is it only to Apple TV and 3rd party speakers?
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.