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.
But while I've got you thinking about monetizing your site, allow me to make a shameless plug for something else we’re doing at Yahoo! -- take a look at the Yahoo Publisher Network BETA. Revenue, relevant ads, no hassle. Easy. Sign up today for your YPN beta invite.
Check out my follow-up post at the Zvents blog for the rest of the story.
The Zvents team is now going to get back to work building product. Google, Yahoo, Scoble, we love you all. Peace and APIs.