Adding a mobile ads to monetize your mobile applications is a quite easy task, when you're compiling your applications locally. And there were many articles written about this (like the one at PhoneGap Wiki).
But, when you compile in-the-cloud, using PhoneGap Build only, things are getting much more complicated.
If you have setup your local development environment and you compile your mobile applications in local PhoneGap, then you have access to Java code of the container for your HTML 5 application. You can modify it to reduce container screen dimension and place some ads in your application. This let's you choose among many mobile ads networks, which provides SDKs for displaying their ads.
Thus, MobFox seems to be the only solution for PhoneGap right now.
And if any of mentioned (here or below) does not satisfy you, you may consider programmableweb.com, a promising API directory, that I ran into by a complete accident. It let's you browse really impressive number of external API services and browse it by
Data format and so on.
So, for example, here is a list of API providers, currently stored in programmableweb.com's database, filtered by
Data Format=JSON. Something, I've been looking for.
I haven't tried any of them, so above answers, with concrete examples are still valid (and new ones are most welcome). But I wanted to mark it out, that this could be a good start point for seeking an API to this or any other purpose.
Update: I've tried some of above and many entries are really, really old (orginating in 2006) and thus, of course out-dated or not-existing at all. Even so, I advise to use it as a good starting point, because there are many pearls among old junks there.