I found out that you can use a simple (and more intuitive) URL format to link to various items on the iTunes Store, at least for some categories. For example:

• Artist or App Store developer: http://itunes.com/ArtistOrDeveloperName
• iPhone/iPod touch app: http://itunes.com/app/AppName
• Movie: http://itunes.com/movie/MovieTitle
• TV show: http://itunes.com/tv/ShowTitle

URLs are case-insensitive. For multi-word titles, you should remove the spaces, for example, http://itunes.com/kanyewest. I couldn't, however, figure out a more intuitive way to link to individual songs or albums.

[robg adds: This worked for the handful of movies, artists, and App Store apps and developers I tested it with. I had mixed luck with TV -- you can link to series, like 30 Rock, but I couldn't figure out how to link to a particular show, or to any of the CSI series, for instance.]