iOS 4.3 is now available for download, and along with it comes iTunes Home Sharing -- the ability to stream your entire iTunes library to your iDevice over Wi-Fi. Previously, you could use Apple's Remote app to control and play an iTunes installation on another computer, but this is different. With iTunes Home Sharing, the audio from files stored elsewhere actually comes out of your iOS device, so you can listen to iTunes while on your treadmill, or stream a movie to your iPad on the couch.
Note that Home Sharing requires an Apple ID, and all of the standard license requirements apply. One fun way to use this: try streaming music out to your dock in the bedroom. Your complete iTunes library is now available anywhere in the house!
Home Sharing is found in the iPod settings on your iOS devices.