From the iTunes 6.0.5 release notes:

Note: After purchasing music from the iTunes Music Store with iTunes 6 or later, you also need to upgrade your other computers that purchase music from the iTunes Music Store to the latest version of iTunes.

Is it just me or does that imply that they’ve updated their DRM again? I can’t think of any other reason why newly purchased music would be incompatible with older versions of the software.

In any case it neatly highlights the extent to which music bought from the iTunes music store is locked to Apple’s proprietary software. You did realise if you want to keep playing the music you bought from them you can never buy an MP3 player other than an iPod, right? And if you decide to ditch Mac OS in favour of, say, Linux you’re shit out of luck too.