Did a little digging, found this on voltage. I am not using a resistor but it appears I should be. I have not however blown any bulbs yet using unregulated DC.
As a more direct answer to the question: there is THEORETICALLY no limit to the voltage you can use to power an LED... you just need to add suitable current limiting, usually in the form of a resistor.
If a given LED has a nominal current of 20mA, and has a forward voltage drop of 4V, then to power it with a 12V supply, using Ohm's law (V=IR, or R=V/I), you'd get R=8/.02, or 400 ohms. You could power it directly with 120VAC by using R=56/.02 (since you're only powering it with half the AC wave; the other half is "reverse" voltage and will be blocked because, well, it's also a diode), giving you 2.8k ohms.
The other consideration is power dissipation: 56V at 20mA is 56*.02=1.12W, so your typical small 1/4W or 1/2W resistor would burn up in short order in that circuit. In that case, you'd need to use some kind of either voltage-limiting or current-limiting power supply.
But yes, with the right circuit design, you could drive an LED with almost any common voltage.