Apple’s cloud-based music service might be just around the corner, but in the meantime, you can have complete access to your iTunes library on any iOS 4.3 device via your home’s Wi-Fi network by setting up Home Sharing. In the iTunes Preferences window, make sure you have Sharing turned on, then go to the Advanced menu and turn on Home Sharing.
Once that’s done, go to your iOS device’s Settings app, tap on iPod, and scroll down to the Home Sharing section, then enter the same Apple ID and password that you used to setup Home Sharing on iTunes. From then on, whenever you access the iPod app while connected to your home network, you’ll see an option for Shared under the More tab.
Select your library, and you’ll have full access to every song, album, and playlist you’ve got, turning every iPod/iPhone/iPad speaker in the house into a mini music server. Note that if you have a second-generation Apple TV, you can likely skip the iTunes steps and go straight to your portable device.