The review unit at The Verge already has a small crack, from when it was accidentally knocked off of a table. And Android Central, in their review, said they were already seeing hairlines scratches on the back. I was also in a T-Mobile store today looking at their demo unit, which had a few hairline scratches on the back (even though they just got the phone today).
The glass back is one part of the build of the phone that I a bit worried about in the long run (after all, it didn't work out so well on the iPhone 4).
Thanks for any comments.