"Yahoo! Mail offers an incentive for developers to build applications using the full functionality available for premium Yahoo! Mail accounts. Specifically, Yahoo! Mail provides a commission of $10.00 for every new Yahoo! Mail Plus account referred by developers."
It would be interesting to see how many developers would see this as an incentive to build an application around the Yahoo! Mail Web Service. It would be even more interesting to see the conversion rates overall and per application.
On more practical side, maybe I could find couple of hours this weekend and throw in together some WPF/WCF app to check my Yahoo! email. :-)