I am not sure about this, but it seems that Google's webmaster trends team is sending out snail mail letters to some Sitemap users in the Google Webmaster Help forums. I believe the letters read:
Help webmasters create great sites.
I think this is a gesture of appreciation to some of the more active members in the Google Groups support forum. Again, I am not 100% sure about this, because I do not have a picture to prove it and the language used in the Google Groups thread is a bit cryptic, but I think I am right about this.
Forum discussion at Google Groups.
Update: I have been informed that these are not letters, but rather T-Shirts that have the XML above written on the back of the shirt. I hope to acquire a picture of this T-Shirt. And here is a picture provided by John: