Does the LED indicate the actual delivery of current to the plugs, or just the time for which you should wait before turning the starter on; or, in sequential terms:
You put the key in and turn, the yellow LED goes on for a bit, then it goes out, and you start.
If you instead turn the key off, then on again - no LED, but do the glow plugs get current?
First guess is right. It shows the recommended time you should wait. On the older diesels (rabbit and mk2 golf), it should continue to glow for X seconds (not sure exactly) after the light goes out, or until you turn the key to 'start.'
Bentley has more information under troubleshooting.
The time the glow plug light is on varies with environmental conditions and, I believe, turns off only once the thing is hot enough. If you switch off and on again, it can take a second or two to get the 'ready' signal back.
My '97 Jetta, 161k miles, starts just right after the light goes off every time; in the winter it coughs and sputters every time I start too soon.
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