Want to use AirPlay, but don't have an Apple TV? There's hope for you yet. TUAW's Erica Sadun put together an app which'll allow your Mac to stream media from an iPhone by pretending to be an Apple TV.
The app is called AirPlayer and currently an ad-supported alpha version (meaning that it's really more of a proof-of-concept than something to rely on at this point), but it sounds pretty decent already:
What AirPlayer does is create and advertise a custom Bonjour AirPlay service that pretends to be an Apple TV. Bonjour is Apple's zero configuration networking solution for allowing devices and applications to communicate with each other over local area networks. When Apple created AirPlay, it basically set up a new way for Apple TV to interact with iOS using Bonjour communications.