Portal would be extremely well suited on the Wii.
The beauty of the Source engine is it's scalability as HL2 on XBox proved with it's near-enough identical port, and with time would probably prove a better experience on Wii's hardware, especially with some of the new visual features the latest HL2 titles added in the engine.
Only killer though is the game's depth - or lack of! It would need aload of extra content besides the advanced levels / challeneges to prove enough sustenance for a standalone Wii game.
Other than that I hope it happens, Portal has been one of the titles of the year - undoubtedly :)