Interesting. How do they protect against clickfraud, though? Paying the user seems to me (somewhat naively, because I'm not super familiar with clickfraud) to increase the incentive for abuse, since you don't have to run a malicious website to do it.

One of the obvious advantages of the Gmail model seems to me to be that free email is less fungible than cash, though of course abusers resort to spamming and other practices to monetize the resource.