I just received an invitation to Google Music. I noticed that it's possible to edit the meta data for tracks through the website including the album art. If I wanted to switch to another music hosting service later is it possible to download all my uploaded music with updated metadata?
You can visit google.com/takeout to see the services which Google currently lets you export your data from.
You can also visit the "Data Liberation" website to see Google projects where data can partially be exported.
However, neither of those list Google Music at all, and a random comment says it doesn't work yet, so the evidence is that no, you can't.
(Not being a US resident, Google Music isn't available to me, so I can't say for certain.)
Here we are almost 5 years later and the answer is now... still no.
I just tried this out by downloading a song that I had uploaded and added a comment to. I was able to download it but the downloaded MP3 had no comment.
Aside: It's disturbing that Google limits you to 2 downloads per song. It is, after all, your own music. The limit doesn't apply to Chrome, apparently, but for how long? I'm keeping backups elsewhere.