With the iCloud unveiling yesterday, Apple also announced that a new version of iTunes and a beta version of iCloud would be available now (with the rest rolling out in the Fall). Users have been reporting iTunes 10.3 downloads becoming available over the past 24 hours. Now, it appears to be available for everyone.
Keep in mind only some iCloud services are available now (and they're still considered beta). The rest will be rolled out this Fall. You should already see changes to the App Store on your devices as those were rolled out wireless yesterday.
iTunes 10.3 will let you do the following right now -