Our position has always been to allow usage of the Yahoo! Maps APIs free of charge for non-commercial use, as well as commercial use granted on a case-by-case basis. This is defined in our FAQ which also has instructions for how to contact us should you want to seek permission for commercial use.
In general, if you are displaying mashups featuring Yahoo! Maps on your site or application and you make your stuff available for free to users, you’re welcome to use the Yahoo! Maps APIs. This is true even if your site is supported by ads — even ads from other vendors.
Without a doubt, we still want you to sign up for for Yahoo! Publisher Network to display our ads on your site (for which you will receive a cut, of course), but we’re not going to ask you to use YPN unless your own stuff isn’t free.