iTunes 10.3 is now available in the UK from the Apple website. It doesn’t seem to be available through the software update as yet. You can get it here -> http://www.apple.com/uk/itunes/download
Apple have today released (already) version 10.3.1, to possibly fix a device sync’ing bug. There is no official word from Apple what this update does.
The iTunes 10.5 Beta doesn’t seem to be much different. It’s been polished a bit, and the only thing of note so far is the slightly different volume control slider.