OK, here's my completely unscientific comparison...
I got two Airs (one is for a friend), same configuration, same order. They shipped from the same place and arrived at the same time. The serial numbers have one different character. They are identical down to the geekbench score, except that one has a Samsung panel, and the other has an LG. Go figure.
I took a bunch of photos, with and without calibration by eye (on both of them at the same time). Everything was done side by side. Don't ask which shots are calibrated... I don't know anymore, but I included the system preferences app in some to make it clear.
They were taken with an iPhone 4 in a dimly lit room. Deal with it.
See the photos here (edit: oh yeah, Samsung is on the right):
https://www.dropbox.com/sh/9qxinbz1bpfsp39/hWP6K-5Tcq
My observations:
Yes. the Samsung screen has more contrast, but IMO, a bit too much by default. The LG shows a bit more detail in the darks, but less pop in general. The LG's white is *ever so slightly* darker when side by side, but not noticeably otherwise. Calibrated, they are very close, but the Samsung seems to have more trouble with very smooth gradients (such as window shadows) in either case. For example, the login screen with a solid gray background shows some extreme banding on the Samsung (not so much on the LG). That bugs me a bit. The Samsung is not "sharper" as some have said (I assume that's just a misunderstanding of the terms contrast vs. sharpness).
The Samsung has darker blacks, as you can see with the black screen when the lights are out, but the LG seems more even overall as you can see in many shots, but mostly on the gray login screen and dashboard background. Blues seem a bit richer on the Samsung, but as with contrast, almost too much.
My opinion:
They are both very nice, and I seriously can't decide which one I will take for myself just yet. If I were to use it mostly for watching movies and stuff, I might prefer the Samsung just because of the slightly darker blacks, but possibly the LG since along with smoother gradients, compression artifacts don't seem to stand out as much. For black on white text, the Samsung also seems a bit better when side by side with the LG, but it wouldn't bug me if I couldn't compare, and neither is amazing after seeing a Retina MBP. If this is at all a companion for photo or graphics work, I think the LG may have an edge, since it doesn't lose as much detail in dark areas, seems to be able to handle gradients better, and is more consistent across the screen.
I will be sleeping on it and comparing again in daylight tomorrow, but honestly, I kind of wish I didn't get to compare the two. As picky as I am, I would have been plenty happy with either.