Ars asked how this works and Google said, "The app code is all running on top of the Chrome platform, specifically inside of Native Client. In this way the ARC (Android Runtime for Chrome) apps run in the same environment as other apps you can download from the Chrome Web Store, even though they are written on top of standard Android APIs. The developers do not need to port or modify their code, though they often choose to improve it to work well with the Chromebook form factor (keyboard, touchpad, optional touchscreen, etc)."
There is no porting required; Google has built an entire Android stack into Chrome OS using Native Client.
You can also tell Google what Android Apps you'd like to see most: http://www.chromebook.com/newapps