Microsoft has its own ad network with which app developers can integrate -- but it's definitely strange that Google has yet to announce any kind of ad network for Windows Phone 7. An ad network with comparable features to AdMob which provides a Silverlight frontend is going to be huge when someone decides to build it.
If you want to hack AdMob ads into your Windows Phone 7 app, you certainly can do it, but there is no native support for this platform, and no indication on the website that they are even thinking about it.
The Microsoft alternative is easy enough to use -- however, it's currently impossible for the average advertiser to put their ads on the network, and even more impossible for the average app developer to run in-house ads to cross promote their other apps.
Want a billion dollars? Be the first to create an awesome ad network for Windows Phone 7, and wait for Google to buy you.