There are a couple options for cloud storage and access of your music with Amazon Cloud Player and Google Music. Apple will soon be launching their iTunes Match service and according to TiPB the service was offered in beta form to developers last night. For $25/year iTunes Match will let you upload a maximum of 25,000 songs to your account for access from all of your Apple devices. This is NOT a subscription music service like Spotify, Slacker, Zune Marketplace, but simply a cloud storage and access solution.
If you have a large music collection and an iPod with large capacity and no other iOS devices then this doesn't make sense for you. This is for those with large music collections and multiple Apple products who don't want to store their music on every device. The iTunes Match service lets you both stream music and download it for offline playing. iTunes Match should launch for everyone when iOS 5 launches in the next month or two.