Windows Phone 7 is a completely new system. Microsoft is taking a risk ditching the mass of (admittedly poor quality) Windows Mobile apps out there. They need developers to step up to the plate to make the system viable. While there are about 1000 apps available as the platform launches, developers are finding some things to complain about. One of the first issues: the SDK does not let developers have full access the camera hardware.
Developing an app dependent on a particular piece of software relies on the SDK having the necessary APIs. Right now, image sensor access on Windows Phone 7 is limited. The makers of popular apps Layar and Fring have both put their WP7 plans on hold due to this issue. Microsoft has indicated that they are happy to have developers use the camera for picture taking, and APIs do exist for that. But the sort of "viewfinder" capability that video calls or augmented reality would need is not available.
It's unclear if Microsoft plans to add this feature in a future version of the SDK. It might be a small omission in the grand scheme of things, but when Microsoft is starting at a disadvantage, they shouldn't be limiting developers.