WordPress.com retains a fairly strict anti-advertising policy. Here is a snippit from their TOS.
“AdSense, Yahoo, Chitika, TextLinkAds, and other third-party advertising is not allowed here at WordPress.com…. 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.”
However, they’ve recently announced their own WordAds advertising option in partnership with Federated Media. If you’d like more flexibility with advertising then you may want to consider one of the paid hosting options where you can install your own plugins that support Google AdSense, Bing, etc..