Inside AdWords blogs about the new test (after the CPC site targeting ads) which allows advertisers to pay for actions (e.g. a completed purchase). I think this feature was reported before and was tested among larger advertisers on invitational basis. This is the first time it seeks outside testers. Like the CPC site-targeting ads test, this test is only available to advertiser in the US, which means I will have to wait for a while ... sigh ...

http://adwords.blogspot.com/2007/03/pay-per-action-beta-test.html