It seems that although Sitemaps don’t get a new site spidered any faster,
once a site is already in the index, or after Googlebot has been through once,
the Googledance sucks up the whole site — all 600+ pages of each new site I submitted a sitemap for are in Google today. The site I didn’t use a sitemap on has only 192 pages indexed out of nearly 500 pages on the site.