Basically what the feature is, is that people can publish an XML document which the Google spider will look for and then use to index your page. In the XML you list all of the pages of your website and how often those pages are updated.
The way that I look at it is that this just makes the job for Google easier. Making the users do the spidering for them.
However, I also see that this could be useful. What if, instead of just leaving the sitemap hidden on your server, instead you create a script which creates an attractive map to your website. And what if someone were to create a Firefox extension which looked for this sitemap.xml. Then you could have a cool sidebar which shows all of the pages in the site. There would be no more mystery navigation, everything that the designers want their users to see would be all laid out in front of them.
There is actually a few people in this Slashdot discussion who are talking about the same thing.
This is defintiely an idea to chew on. I may look into developing this myself. If anyone out there goes at it, i'd love to hear about it.