A WebmasterWorld thread has discussion about a possible new bidding feature in AdWords. Honestly, I am not sure how new it is, but it basically allows you to set a target CPA (cost per acquisition) and Google will try to optimize your bids to achieve it.

The advertiser wrote that he was presented with this option after logging in to manage his AdWords account. Google told him:

I believe that this was originally a limited test, but if you run Google Conversion Optimizer you can opt for this target CPA now or soon.

Forum discussion at WebmasterWorld & Google AdWords Help.