If your blog is hosted on Wordpress.com, you cannot monetize it with Google Adsense, Text Link Ads, PayperPost or other advertising programs as that’s against the rule book.
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Sponsored / paid posts including PayPerPost and ReviewMe are not permitted. Sponsored / paid links are not permitted. Text ads are not permitted.
Adsense, Yahoo, Chitika and other ads are not permitted to be added by users. Adverts that may be inserted when using an external blogging program will be blocked.
One discreet link to Amazon per blog is okay, but if the primary purpose of the blog is to drive traffic to affiliate programs that’s not allowed. If you’re not sure, contact support.
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However, if you have a self-host version of Wordpress downloaded from Wordpress.org, you are free to modify templates or integrate advertising code inside.
Now some hope for people who have blogs on wordpress.com - you will be able to add adsense to your blogs soon.
According to Guardian, Wordpress founder Matt Mullenweg plans to allow users to add Google’s AdSense to their blogs. It may be a paid upgrade and users will have to pay around $15 a year per blog to see Google Adsense ads running on the their wordpress.com blogs.