I run the same app on Windows 8 and have not seen the advertising there. Don't know if it is because of some special deal not to run advertising during the trial period, or I just haven't run Win 8 enough to have it start.
I suppose time will tell. Maybe I should find an App I am willing to pay for and see what it does!
I wonder if the Tablet announcement by Microsoft will mean anything.. if it happens.
I can answer you for Android OS and iOS. For both, most free apps are paid for by some sort of advertising. We publish an app for Android for Windows7Forums.com and could not continue to contribute to it, license it, and send it out without some form of advertising. The amount of revenue it makes is minuscule, but it is enough to pay for licensing and development fees. Most apps on Google Play (formerly Google Marketplace and the iTunes App Store that are free will run ads to pay for their development). I suspect most Window apps will too.
We could offer a $1 charge for an ad-free version of our app, but its unlikely many people would buy it. The ads are very unobtrusive and do not degrade the performance of the software. For us, it is better than having no app at all. So in the future, yes, there could be quite a many very good Windows Runtime/Metro apps that are provided for free paid for by advertising.