So if you are running Skyrim in SLI, your HDR (high dynamic range) is broken.
And has been since release.
Still broken in version 1.5 (just released today).
This impacts a lot of things in the game - colors will be much less lush in spots, and especially lighting. Any time you are seeing 'god rays' (crepuscular rays) in caves, transitioning from one light source to another, looking at magic effects, etc...all that is going to be much more 'muted' and less impressive than the designers intended.
A quick way to check is to go outside and look at the ground, then quickly at the sun. If you see this:
...followed immediately by this:
Then HDR is broken for you!
What you should see is something like this for the ground (so far, so good):
...then when you look right into the sun...ZOMG, HDR BRIGHTNESSES I CAN'T SEE!!!
...oh, but, wait, cool...eyes adjust, all is well. I wonder if I...
...look at the ground...ZOMG THE GROUND IS GONE INTO SOME BLACK ABYSS OF...
...no, wait, eyes adjust, all is well.
So that's the effect you want. Nearly entirely-whited-out screen when first looking into the sun, nearly entirely-blacked-out screen when then looking right back at the ground. Seeing that means you are getting the true color depth (most often and most impactfully noticed in lighting) in the game. And, of course, that HDR is working.
As noted in the intro, there is apparently some kind of incompatibility with SLI that hasn't been resolved, yet. So...by default, you aren't seeing what you should be.
Fortunately, a workaround is easy. Simply enable 3d vision...even if you don't use it in the game. First off, make sure the nVidia 3d vision driver is installed.
If you don't - it's pretty easy to add. Just download the latest WHQLs, and run the installer. Make sure to select the 'Custom' option...
...and enable the '3d' drivers.
That done, you'll have a new option in your nVidia control panel...the "Set up stereoscopic 3d" section. Just open that up, and check the box for 'Enable stereoscopic 3d'. You will also want to (assuming you don't actually want stereoscopic 3d in your game) to check the box to 'Hide stereoscopic 3D effects when the game starts'. You may have to go through a quick setup wizard for the 3d effects, but that can be done with or without 3d glasses (just pick different shapes...not the 'blended together' shapes...on the first screen you see that offers that choice).
That done, you get HDR on your SLI system with no 3d-vision effects. Works on any monitor, any system!
Now, yes, there is a performance hit to doing this vs pure SLI, but...eh. What can you do?