There are apparently some burn fixes for Superdrives but network sharing is now confined to your own subnet. When you download through Software Update, your old copy of iTunes 4.0 is patched. If you want to retain sharing, I doubt you can run the new version on the same machine as the old.
From the readme:
iTunes 4.0.1 includes a number of performance and network access enhancements, and only allows music sharing between computers using iTunes 4.0.1 or later on a local network (in the same subnet). If you have an Apple SuperDrive, you can archive your song files to a DVD disc as well as burn playlists to standard audio CDs to play in your car or home stereo.
Digging a bit deeper, the Shared Playlists I'd previously accessed are still visible in the Source List of iTunes but are greyed out. Selecting them brings up a dialogue saying that the selected library is not compatible with this version of iTunes. Since iTunes is seeing these playlists, they are likely on the same subnet as my Mac but the other users will need to run 4.01 in order for them to be accepted by a 4.01 user. In other words, if you upgrade to 4.01, you can only see other 4.01 user playlists, even if they are on the same subnet... Bummeroony.
Seems that this was the price Apple had to pay to keep the record co's happy. Wonder if there will be more prices to pay down the line??
And no, 4.01 does not grant access to Canucks to the iTMS (but you can bet when it does come along, you'll need the latest version of iTunes....).