Thanks to a friend, I came to know about one advertiser who is looking for blogs with PR2 and above who would like to buy links on the sidebar. They are paying USD50 per link. So far, a blog gets a minimum of 2 links.
The thing is, I submitted 40 blogs and only 2 blogs met his requirement. One of the reason of his rejection is forge PR.
I know lots of bloggers these days will buy expired domain with PR instead of starting a new blog from scratch. All because advertisers will only advertise in PR blogs. Due to this, many blogs with forge PR sprouted.
To date, I still have no idea how they do that and how to check whether the PR are valid or invalid.