Idea for a visual mod if anyone wants to try it.
Not sure exactly how doable something like this would be, but I thought it might be an interesting task for someone bored, to try to take on.
As we all know, objects go through portals seamlessly. On levels with awkward lighting though, this could lead to some strange looking cut lines, especially when moving an object from a light room to a dark room.
Well I'm sure regardless of some mild performance slumps, and interesting mod could consist of copying lights from 1 side of a portal to another side so that light seems to travel through the portal.
Take this picture example using a spotlight.
Now of course, to keep with performance, there might be something such as a limited amount of lights that are copied and perhaps a distance.
A mod like this would lead to some nicer looking object transitions between portals and actually might even go as far as to add an entire new aspect to puzzles.
For example, using portals to light up dark rooms! 
Its a great idea, and I'm sure most of us thought about it whilst playing, I know I did, but it would probably lag like mad.
I was thinking of this the over day I thought of a map were you would have to use your portals to see where to go
I'm not even attempting to think this through entirely but it might be possible to simulate something to this effect with a func_portal_detector switching on/off lights. Have no clue how you'd actually go about doing it though.
Well in Portal, when you place say for example, a Box into a portal. Theres actually 2 boxes now. The half on your side and the one on the other half. Theres a split down the middle to make them both align correctly.
Essentially thats how the lights would work. A copy of all near by lights would be created on the other side.
To keep the lag down, the game might limit to lights only within say a 64 point distance and limit it to maybe 2 or 3 lights at a time.
Well, yeah. Thats if you want to do some actual coding, which might get difficult. Figuring out some genius scheme turning lights on/off would also be somewhat difficult, but at least all the tools you need are already in hammer (presumably) to make it good enough.
It would be cool if someone took the time to code it up though.
Only problem is that the lighting is precomputed when you compile the map so I don't think its possible.
Normal lights can be turned off and on, very easily infact.
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I hadn't thought about this before...
BUT it would make new puzzles. Because in Portal, you obviously have no flashlight. But you could bring light into really dark rooms using portals.
Hmmm.... Very nice!
EDIT: Just saw original poster pointed this out already. I'm apparently just expressing my agreement.
lights can be turned on and off its true, but in order for this to look good its going to need to be a spotlight, so you would have to place hundreds along all the walls and at every angle (since the angle of the light going into portal A would change how it would come out of Portal B)
Right now a portal is basically putting a camera behind its connected portal to allow you to look through, it doesn't pass any information back through. Anything's possible, but my bet is you have to wait for Portal 2 