A Bethesda employee currently designing levels for the developer's forthcoming RPG Fallout 3 has been playing around with Portal map creation.
Daryl Brigner has created a level for Valve's first-person puzzle-'em-up called Ren_Test2.
We've not zapped holes in the map ourselves as yet, but judging by comments that have been spouted it's a clever design but will test your Portal powers to the limit.