While here in Austin at 360iDev we actually received a number of questions from developers about monetization on the new Windows Phone 7 system. We passed along these questions to our contacts at Microsoft and have the following answers.
Microsoft offers a free ad SDK which you can download from this link: http://advertising.microsoft.com/mobile-apps
Microsoft is operating the ad program and your check will come from them. InMobi is just one partner who is providing Microsoft with ad inventory.
We don't offer one at this time. Microsoft does allow third party tools and in the future this may be something the InMobi will offer.
Developers can access tools to build apps for Windows Phone 7 from this link: http://create.msdn.com/en-US