It's a bit obscure, but iTunes actually offers email notifications for new releases by artists you select. To add an artist to the list, go to the artist's page within iTunes, and click the Alert Me link in the right sidebar, then click Manage My Alerts.
You'll see the artist has been added to the list (it may also contain artists you've previously purchased music from). Check the Send me email alerts about artists I've previously downloaded box to (surprise) get email when they release something. To add more artists, click the Alert Me on their page, then click OK.
If you don't want to use email for this, iTunes offers new release RSS feeds. You could filter these with something like Yahoo Pipes to get a feed with only the artists you're interested in.
It's worth noting that these both display new releases when they come out, not upcoming releases. As far as I can tell, Apple doesn't publish any upcoming release data, except for the pre-orders page, which doesn't seem to contain all upcoming releases, just featured ones.