Apple announced iCloud today at WWDC and with it the ability to re-download your previously purchased music.



From TUAW:

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.]/quote]

  iTunes moves to the iCloud, re-download of purchases now possible