Apple announced iCloud today at WWDC and with it the ability to re-download your previously purchased music. Re-downloading of 256 Kbps AAC tracks is now possible for the first time from iTunes, meaning that you can purchase your music once and download it to every device you own (up to 10 devices total), without any additional charge. Like apps and books, music also doesn't count against your 5 GB iCloud storage space.
You simply search through your purchased music in iTunes and hit that cute little cloud icon to initiate the download.
What's more, Apple has included Automatic Downloads for iOS into the mix meaning that free and purchased music will automatically pushed on any device you have connected to the Internet. Buy it on your iPhone and have it pushed to your iPad. Sim