"With regard to existing applications: today’s Windows Phone applications and games will run on the next major version of Windows Phone. Driving application compatibility is a function of Microsoft’s commitment to its developers. Regardless of what we release in terms of new developer features and functionality, we have made a large investment in protecting your existing investments."
While this isn’t the first time we’ve heard that Windows Phone 7 apps will be supported in Windows 8, it is definitely reassuring to hear it from the source for both developers and app purchasers who are invested into the platform.
source: Windows Phone Blog