AdSense, Yahoo, Chitika, TextLinkAds, and other third-party advertising is not allowed here at WordPress.com. If you would like to run ads on your blog, one of these options may work for you:
We have a feature called WordAds that lets WordPress.com bloggers with moderate to high traffic and appropriate content turn on ads and earn money from their blogs.
You can run any ads you’d like if you manage your own WordPress installation. More info, including hosting recommendations, can be found at get.wp.com.
WordPress.com VIP sites are permitted to run ads.