Unlike CRTs, LCD displays simply get a bit dusty (disregarding what happens to anything exposed to smoke), so all you usually need to do is lightly remove the dust-a slightly damp, lint-free cloth is perfect for this (cloth diapers, anyone?). CRTs attract dust electrostatically-it's because of how they work-so you need to work harder at removing the dust.
Smoke, especially cigarette smoke, is somewhat sticky, and leaves a film that both darkens with time and attracts/adheres dust. It often takes a fairly serious solvent-based cleaner to get this stuff off, and such products are not very gentle on the plastic coating of an LCD display. As a rule, it's good to keep smoke away from any computer, but since LCDs are less physically sturdy than CRTs, it's particularly important not to let them get coated in this gunk. Better yet, don't expose them to it at all.<</end of anti-smoking rant>>