Google's latest labs project is Google Music Trends, a service which tracks the listening habits of users of Google talk and produces aggregate statistics about their listening habits. This is taking pretty firm aim at last.fm and LaunchCast Radio which already provide this feature.
It's interesting, but like a lot of Google's more recent efforts, I don't think this is going to change the market much. GMT has the same failing of launchcast, which is that it provides lots of aggregate and whole-universe stats, which are useful and interesting to owners of the site, but provides end-users with little or nothing in the way of personal statistics. The users of last.fm use the service because it suggests music for them and it generates detailed weekly and monthly listenings charts for them (and easily lets them post these to their websites).
This gives people a very clear personal reason to sign up, and then later they catch on to the social aspects and global aspects. What's the incentive to sign up to GMT, other than to lose a little bit more of your privacy to Google?