When using an LCD display with a DVI cable, does the refresh rate have any effect? Is there a difference between 60 hz and 75 hz, for example? I thought those numbers specifically described the motion of the raster in a CRT, and would be meaningless for an LCD display. But the software lets me change it. I can't see a difference though.

Side question: For 3D games, does the "wait for vertical sync" feature have any effect in this case? I know what "tearing" looks like on a CRT during a video game, would this same effect happen with an LCD connected digitally?

Yes, tearing does still occur. For the same reason that aiming a camcorder at a tv will show "tearing". The signal coming to the LCD, if at 60hz, will show tearing if the frame rate isn't exactly 60fps.