How well does the new interior lighting system intergrate into a new build? I enjoyed the interior LEGO lighting I saw at Brickcon 2011. How does the lighting system affect the fit of the LEGO elements involved?
Considering the new PF lights are more compact at the point where the light is, I would say it's easier to integrate in creations. They do have a big block on the cable where the two lights are separated, but that leaves you more liberty compared to previous 9V lamps for which you had to have the cable connector on the lamp itself, which took quite a lot of space. 

As such, you may be able to fit PF lights in tighter places, and have less constraints on the orientation (I'm thinking ceiling light with the bulb directed downwards, for example).

By the way, that's an area where a lot of LEGO fans are more willing to toss purism aside and use their own leds and cabling, which can easily be done without damaging a single brick, takes less place, and usually costs less.