The interesting thing, however, is that these are audio tracks. As anybody who sings karaoke knows, it is much easier to sing when you have the lyrics in front of you, which is what you get when playing a CD+G karaoke disc on a compatible player.
Hopefully, this will be the start of a bigger trend. Since Apple sells videos through iTMS, they should start selling graphical karaoke tracks as videos. So when played, you get the graphics you'd see on a CD+G release. I think these would be very popular, especially if they're priced at the same $0.99 that audio tracks are sold at.
Of course, it would be even nicer if iTunes could play CD+G discs, ripping them to H.264 video files. But that may end up being too good to be true.