No, you are not permitted to do that on Wordpress.com blogs.
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.
In addition to AdSense-type ads, please do not use the following services on your blog:
Sponsored or paid posts, including PayPerPost, ReviewMe, and Smorty.
Affiliate or referral links to the following domains: usercash, clickbank, clickhop, cashrocks, payingcash.
Clicktrackers and any promotions of the “I made a million on the internet and so can you” type of advertising (i.e. MLM, network marketing, cash gifting, etc.).
Paid or sponsored post content is also prohibited.