The Yahoo! Publisher Network has one feature that Google AdSense does not have. Yahoo allows you to transfer your earned publisher income to your Yahoo Search Marketing account, to spend the money there.
According to a WebmasterWorld thread, AdWords advisor claims that this feature "is on our radar." But even more so, he/she explain why it is taking so long for Google to implement.
To that I'll add that, for a number of reasons, including regulatory issues with the many different currencies with which one may pay for Adwords (or be payed in by AdSense), this is not nearly as simple as it might appear to be at first blush. ;)
I wonder if this is one of the reasons why Yahoo hasn't opened up YPN to publishers outside of the United States? In any event, this would be a nice feature to add, based on the number of requests I have seen out there over time.
Forum discussion at WebmasterWorld.