Driving a LED from a wide ranging input voltage
Assumption: The requirement is for a "regulated power available" indicator, rather than merely a "mains supply on", since the latter would not be affected by the variable voltage of the power supply as mentioned. 

LEDs are essentially current-driven devices, not voltage driven. As long as the supply voltage is at least as much as the rated forward voltage of the LED (plus any headroom for current regulation circuitry), and the current through the LED is regulated to a desired value, the LED will be lit at a constant intensity. Typical indicator LEDs commonly are designed for 20 mA current, but will work excellently at 10 mA.

The easiest way of obtaining a constant illumination from an LED across a wide range of supply voltages, is to use a constant current driver circuit. 

This can be made using, for example, an LM317 as a constant current source:



Alternatively, use a constant current 2-terminal device such as the SuperTex CL220 or CL2, simply wired in series with the LED. In other words, it is as simple as using a current limiting resistor with the LED, just with one of these parts instead of the resistor.