Apple has an Apple TV apps engineer, if LinkedIn is to be believed. What that means exactly is unknown -- maybe they simply engineered the built-in Netflix and YouTube apps -- but as MacRumors points out:
the internal view that the Apple TV is simply a collection of apps seems to be in line with future third-party expansion.
Since Apple TV (2010) is running iOS, it's probably not a question of whether or not it can run 3rd party apps, but whether or not Apple will open it up to running them.
Again, with AirPlay coming in iOS 4.2, the idea of starting up Angry Birds on my iPhone, iPod touch, or iPad, streaming the video to 50+ inch TV, and using the device as a controller is just too good not to get done.