Announced at WWDC 2011 earlier today, iCloud is not a streaming service, but a syncing, storage, and downloading service. Most reports and rumors speculated that Apple's iCloud was going to be all about music streaming, eliminating the need for local storage. But as it turns out, iCloud will be replacing MobileMe, and will be wirelessly syncing data, docs, apps, photos, mail, contacts, music, etc between devices.
iTunes Store customers will now be able to download purchased songs as many times as they wish, even if they were purchased prior to the upcoming launch of iCloud.
It is unclear how it will all work, what with some users having multiple devices with multiple storage capacities, but I'm sure this detail will be found out when Apple updates its site with more info.
iCloud will be a free service with a yet to be determined launched date later this fall.