I think in some cases google can recognise paid link, but not in every case. For instance if somebody is not enough smart and place a direct html link to a payperpost.com or any other related service i guess Google will think that here is something which must be ignored, but must of the bloggers can be negotiated to write review about your site with contextual links and nobody will know about the deal behind the curtain so this kind of links are far not so easy to recognise. Of course these posts must be ordered only for the traffic benefit
