Beware Google's new Pay per action. Far from eliminating click fraud, this creates a new type of fraud: conversion fraud, which is far more expensive. In conversion fraud, site owners who benefit from AdSense conversions fill in forms and submit them to get to the conversion tracking code that triggers the Pay Per Action payment. Because PPA rates are typically more per event than CPC, this allows the fraud to quickly rack up lots in fraudulent charges. And Google has so far been slow in preventing these fraudulent charges and even refunding them, creating an onerous refund request system rather than immediately investigating and refunding them upon being reported to their Adwords support team.

Update: Google eventually refunded us for conversion fraud, so kudos to them.