Thanks to the tip here, I was able to make a smart playlist containing all the DRM encumbered music in my iTunes library.
I remember in the past there was an offer from iTunes Plus that lets you pay a fee to "upgrade" your DRM'ed songs to unprotected ones.
Does this still exist? How do I get non-DRM'ed versions of these songs nowadays?
BTW, I am running iTunes 10.5.1 on Mac OS X 10.6.8.