There is an interesting Google Groups thread discussing how Google handles the <base> HTML tag. The base tag specifies a base URL for all the links in a page. But how does Googlebot or Google handle indexing this tag?
Google Groups member, Webado said you should be "careful using the base tag". Why?
Google's cache adds a base tag set to your own domain root url. If your page is using a different base tag this will result in broken images, missing scripts and css files - all of those end up even mroe broken than it's usual in the cache.
Member, RainboRick said that the major search engines typically "ignore the HTML mark-up in that regard (keyword density)- even MSN."
Forum discussion at Google Groups.