Apple is now offering iTunes 7.2 for download, the update which is 29.6MB for Mac and 36.3MB for Windows is described as follows by Apple;

With iTunes 7.2, preview and purchase iTunes Plus music—new higher-quality, DRM-free music downloads from participating music labels.

Apple and EMI announced in April that it would make DRM-free music available during May, this update allows those songs to be bought and placed into your music library.

Apple has the following section in the iTunes help file dedicated to updating existing songs to DRM-free;

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.