The idea is higher resolutions such as the 1680x1050 1920x1200. Reason being is because of the scaling. Picture your monitor as a transmission. Let's say you have a small 17" monitor that displays at 1280x1024. It's pretty much a crappy broken transmission with 2 gears that your video cards (big block motor) is displaying on. So your SLI'd cards can easily power the best effects at 1280. But lets say you upgrade your transmission to a 6 speed like a 24" monitor aka 1920x1200. Now your SLI'd cards will be able to take advantage of their power and push the frames out. So essentially it's all about scaling. At lower resolutions you'll have too much power and are just wasting money. But at a higher res you'll actually utilize both cards to word hard. In reality you should get higher FPS at any res with SLI. It's just that you won't know it compared to the same single card on a low res. So I would guesstimate that SLI starts to really shine at 1680x1050 and above. Like I said though you'll still get a small benefit compared to single cards at lower resolutions.
I think it also depends on the complexity of the games engine...you'll start finding the limit of one video card much sooner (@ a lower resolution) on say....crytek's new engine. So your higher percentage gains in performance will theoretically kick in sooner.