PJ Hyett has discovered that his out of date google sitemap was the reason for his drop in traffic from google. So if you created a sitemap when they first came out and forgot about it, you either want to keep it updated (creating it dynamically would be a good way to go), or remove it from google.

I went through the whole drawn out process, added my site to their service, and completely forgot about it...
I had a feeling that this was why my site stopped appearing on Google... I promptly deleted the link to my site on Sitemaps.
Sure enough, after checking this afternoon, this site appears again on Google when you search for my name. My advice to you is not to use Sitemaps unless you stay on top of it, who knows how much search engine traffic I've lost.