Correction: I originally posted that Qtrax uses MusicIP. According to a PR representative from that company, Qtrax has no deal with MusicIP--the companies have talked, but no deal has been signed. Apologies for not double-checking all my facts.
I was finally able to get the Qtrax 0.2 beta client, and it's clearly based on Songbird.
Songbird defies easy summarization: it's an open-source project, based on the Mozilla platform, that intends to ease the creation of digital media apps. The basic app is a straightforward music library organizer and player (some of Songbird's founders worked on Winamp), and Songbird offers resources for developers to create customized versions of this basic player (think APIs, documentation, sample code, a loose license, and so on). It's an intriguing project, but I hadn't seen any compelling reason to download it.
The experience is akin to using a skinned version of Firefox: the "browser" appears in the middle of the screen, and defaults to a Qtrax page that offers featured artists, such as Foo Fighters and Amy Winehouse. Surrounding this screen are various other UI elements, including the all-important advertisements. From the home page, you can register via a link on the upper right-hand side of the page (see screenshot), and once you've confirmed your registration via e-mail, you're ready to use Qtrax's search engine to find songs. These songs aren't stored in any Qtrax database.
Qtrax found my test case, UFO's "Love to Love"--about 10 different versions, in fact--but the download button gave me an disappointing but not surprising message that downloads are coming soon. Apparently until the licensing deals are worked out, there's no there there.