Now you can make money with your Wordpress.com blog

In the past, you can't earn advertising money from your blog hosted on WordPress.com

Not anymore for now.

You are given a chance to feature WordAds - an advertising program similar to Google Adsense.

WordPress.com has partnered with Federated Media (FM) to offer WordAds, and only publicly visible blogs with own domains will be considered for this program. Selection will be based on level of traffic and engagement, type of content, and language used on the blog.

You can find more infos on WordAds from WordPress.com site.