Apple released iTunes 7.2 updated in the Mac OS X Software Update tonight, which offers support for "iTunes Plus", Apple's new DRM-free $1.29 offerings announced in April.
Despite the software update, the Apple iTunes Store itself does not show any DRM-Free music currently available for purchase. The DRM-Free launch will likely occur later this morning (Wed, May 30th).
From the iTunes Help, it appears you will be able to upgrade your existing songs to the iTunes Plus (DRM Free) version:
The iTunes Store also offers songs without DRM protection, from participating record labels. These DRM-free songs, called "iTunes Plus," have no usage restrictions and feature higher-quality encoding.
The first time you buy an iTunes Plus song, you specify whether to make all future purchases iTunes Plus versions (when available). You can change this setting by accessing your account information on the iTunes Store.
If you already have iTunes Store purchases that are now available as iTunes Plus downloads, you may upgrade your existing purchases. To do so, visit the iTunes Store and follow the onscreen instructions.
View: Mac Rumours