I just got my invite to Google Music just a few minutes ago.
I created my account then installed the software on my PC and my phone. The PC software scanned my music folder and started to sync my songs to the great Google cloud, and now (10 minutes later), I’m listening to the music collection I’ve collected over the years streaming to my phone.
You can activate up to 8 devices on a single account. Very cool stuff.
I think it will sync your iTunes library too … but I wouldn’t know because iTunes is the most evil piece of software ever, and I refuse to use it.