In addition to lacking DRM, iTunes Plus tracks are less lossy than regular iTunes tracks (256kbps as opposed to 128kbps), but of the four major record labels, EMI is the only one to offer them on Apple's download service.
But (hallelujah!) that looks set to change soon. According to MacUser albums from erstwhile DRM-supporters Warner, Universal and Sony are popping up in the Upgrade My Library section of the iTunes Store. This is where iTunes users convert their existing 128kbps DRM-locked tracks to 256kbps, DRM-free iTunes Plus versions for 20p a track or around 30 percent the cost of an album.
So, while Apple is yet to make a formal announcement regarding the iTunes Plus library growing, it seems the process is already underway. The iTunes Store is about to get more appealing.