Okay, I just tested the lag on my own Samsung's (an HLP model) VGA port. I tested it by using my PC's S-video out, and sending that to a regular CRT TV, while sending its VGA out to the Samsung. Then I ran the frame count video and took snapshots with my digital camera. I changed my PC's resolution to test different resolution signals going to the TV:
720p: This is my TV's native resolution. It lagged 1-2 frames each snapshot, with it tending towards 2 a bit more often. Based on this, I'd say it's behind about 1.6 frames in a 30 fps game, which comes out to about 53 ms. This might be the case with all the HDTVs out there, but this is, I believe, the first actual tested confirmation that the Samsungs *do in fact* experience a bit of lag even in their native resolutions.
800x600: It lagged 2 frames in most snapshots, and occasionally 3. I'd guess it's somewhere around 2.2 frames behind, or about 73 ms lag. I didn't notice any difference between Expand mode and 4:3 mode. I didn't test at 640x480 (480p) because it's pretty hard to even operate the PC at that resolution, but I think it would probably be almost exactly the same.
I'd like to run a test to see how much lag there is running a 480i signal to my Samsung's VGA port through my XRGB-2+. To do that, I'll just take the S-video signal that goes to the CRT and split it, running one half directly to the CRT, and the other to the Samsung's VGA port via the XRGB-2+. My guess is that it will probably drop about one more frame, resulting in 3 frames dropped total (since a 480p signal over VGA already drops a little over two frames), or about 100 ms lag. Before I can do this though, I'll need an S-video to composite adapter cable, and either an S-video or composite splitter cable.