Google must read my blog. No sooner did I post wanting an invite, did I get one. Excellent. So begins my experimenting with Google Music. While classroom application would not immediately appear, and while there is not a huge amount, I definitely see some use.
As a social studies teacher, I always tried to gather some Jazz during a lesson on North Carolina history. Or what about music for projects (Creative Commons, of course!)? Having something like this, a huge depository for your 20,000 songs (I currently uploaded 3,000+), works for me!