In an interesting "case study" presented by Microsoft Advertising, developer Richard Foster is reported to have made a nice $28,882 in four months using Microsoft's Advertising SDK across his 14 apps for Windows Phone:

Richard Foster, once a DOS programmer and now working for the family dry cleaning business and developing mobile applications part-time for his company Edgeway Software, is ecstatic about the results he is seeing developing for Windows Phone 7. Since the introduction of his first app, Unit Converter, in November, Richard has now developed more than 14 apps - all with the Microsoft Advertising SDK for Windows Phone 7 for mobile.

Richard had developed paid apps for the Windows Mobile 6x platform, but since there are a huge number of free apps in the new marketplace for Windows Phone 7, he made the switch to ad-supported apps. For him, this model proved the most lucrative.

Now, in fairness, most developers are not making nearly as much money as Mr. Foster and there's a certain dishonesty in making him a poster-child--plus 14 apps is nothing to sneeze at even if they are things like a "unit converter". However, Windows Phone is still a young platform and it goes to show how just advertising alone, can lead to some serious sustainable income for developers--if ya got the skillz.

And lets not forget that last part: having an app that is coded well, has a nice UI and does something unique is still needed for success. We like to think of '4th & Mayor', 'AlphaJax' and a few others as examples of this talent and good app development.

Source: Microsoft Advertising, via @mechaghost