I have left a comment in moderation on the post linked to by phatpast. That post contains misleading information that does not apply to wordpress.com bloggers like:
As soon as you write something down you can set up an account with, Google AdSense and register your blog with your Google AdSense account.
That post fails to acknowledge any of the restrictions on free hosted wordpress.com blogs. Only very high traffic VIP hosted blogs (paid accounts), and free hosted high traffic blogs that qualify for inclusion in the ADControl Program at wordpress.com can run Goolge Adsense or any kind of advertising at all.