iTunes 10.3 (now 10.3.1) is now available for download through Apple's site. We haven't seen it appear in Software Update as of yet, but it should be there shortly.
iTunes 10.3 introduces iTunes in the Cloud beta with the music you purchase in iTunes appearing instantly on all your devices. You can also download past iTunes purchases, which was first enabled on iOS devices late Monday. The download is 74.02 MB.