It's in the sitemaps page as an option in diagnostics to tell google your preferred domain option - with or without "www". They say : "Once you specify your preference here, it may take some time for changes to be reflected in our index. While Google doesn't guarantee that we'll show your URLs in the form that you prefer, we will use your choice as a suggestion to improve our indexing." Interesting.
Yeah, Sitemaps is now webmaster tools or something like that. Here's my take on it: http://forums.digitalpoint.com/showthread.php?p=1233383#post1233383
It is possible that it is the Google way to fix the 301 redirects from non-www to www so that a duplicate penalty isn't wrongly placed on a website. Since in that area, it states it is not always the case that the different domains are owned by the same person. In some instances, there are owners of a www and a non-www version (I believe for subdomains only this could be the case) and as such could be part of Google resolving the subdomain spam as well as preventing wrongful duplicate penalties.