I'll update it with information I find about this very interesting topic. Particularly if you want to make your own mod, much in the way like you could with the original Portal. So if you want to make one, and really want to make it your own (other than just mapping)...
In UDK you can reverse engineer kismet setups, modeling optimization, world properties of maps, gameplay and new map template set-ups to name a few.UDK already comes with a handful example map files, models, textures and materials. You can open up any of these in UDK ...
they literally stared at me opened mouthed and asked if I were crazy. Those who have played it, might understand, why I was so excited when I heard about theThermal Discouragement Beamsin Portal 2. Seriously, somebody needs to make 100 levels of that shit and turn it into...