With the new iOS4.3 firmware, HomeSharing finally work nicely across all device. This is particularly important if you have an second generation AppleTV in your home network. Will all the new sharing feature you can do two really cool things:

You can finally show any video or pictures from your iPhone on your AppleTV, especially photos and video from your camera roll on the phone. Isn't this the normal use case? You took some pictures of the kids, and when you are at Grandma's house, you can now beam the pictures onto their LCD TV seamlessly. (You did buy the grandparents an AppleTV, didn't you?)
You can, via HomeSharing, listen to any music on your home iTunes library on your iPhone/iPad. Typically in a house hold, you will have most of the shared music and video sitting on an iMac. Now you can listen to any music from there remotely.

How to enable HomeSharing on your phone? There are actually TWO different settings. Go to "Settings / iPod" and enter your Apple ID for sharing. Then you can stream music from any iTunes library on your home network to the iPhone/iPad.

Secondly you can also enable Home Sharing on the Remote app. (you did download that didn't you?) Enter your Apple ID there, then it will automatically find any iTunes library on your network without you having to manually link the remote app to the library.

Tip: The remote app now also can control the AppleTV user interface!