Paul Collins has a great piece detailing the restrictions blocking users from buying tracks on iTunes across national lines. For instance, if you switch your country setting to Japan looking to score some Japan X or Gackt tracks, your US credit card tags you as "illegal," barring your entry into international digital cosmopolitanism. Of course, this is a result of the anal retentiveness of record companies, and the fact that iTunes' price schemes vary from country to country, with users in the UK and Japan getting shafted the hardest.
Like any good border policy though, this has created its own smugglers' economy, where foreign prepaid iTunes Cards are trafficked to, from, and in between different countries using a variety of routes and methods. You can't stop the tunes, man, you just can't stop 'em.