Some users have reported that iTunes 7.2 doesn't see some of their music. The songs in question seems to be ones which were originally purchased from the iTunes store and then later converted somehow to the MP3 format.
One possible fix being reported is to force iTunes to recreate your library via the following procedure.
The new iTunes gives you the choice of paying 30 cents more for music which isn't protected with DRM. Currently this option is only available for songs on the EMI label but I'm hoping more labels follow EMI's lead. The iTunes store gives you the ability to upgrade any songs purchased with DRM for the difference in cost (30 cents).
Another option you may want to investigate is the ability to "complete albums". If you've purchased a few songs from an album in the past and have since decided that you should have opted for the entire album, you can now get the remaining songs for the difference in price between the songs you've already purchased and the cost of the album. If you're interested in this option, be aware that it's only available for a few more weeks.