Google just added a curious new feature to Sitemaps, now renamed "Webmaster Central."  It allows the webmaster to specify whether he or she prefers a site be indexed as www.example.com or example.com.  It is documented here.  This is curious for a few reasons:

1. BigDaddy was supposed to resolve this issue.  In actuality, the problem seems fairly trivial.  It did not surprise me that BigDaddy was supposed to resolve it.  This feature is an admission that BigDaddy is not the end of the troubles in this realm.
2. In my opinion, the current demographics of Sitemaps webmasters are such that anyone willing to create the account and verify it is also probably smart enough to set up a 301 redirect to the same end.  I see this feature, therefore, as only marginally useful with the current demographics.  The demographics may change, however.  Google is now probably attempting to market Sitemaps to a less-savvy crowd.  That may be why they changed its name as well

Unfortunately, it is probably inferior to doing a 301 redirect.  It also leaves it ambiguous as to whether it effects the transfer of link-equity from the other hostname.

It's really not so hard to do a 301 redirect -- at least for Apache; IIS is no more difficult.  Here it is for Apache: 

RewriteEngine on
RewriteCond %{HTTP_HOST} ^example\.com$
RewriteRule ^(.*)$ http://www.example.com/$1 [R=301,L]

So why did Google bother?  It may help inexperienced users, and that's the current marketing push for Sitemaps, I think.