If you're talking about the LM334 or LM317 based circuits, you can get a whole lot more than 20mA. It just depends on the resistor to set the current.
The LM317 circuit can be set with a 60R resistor for 20mA.
The LM334 can be similarly set with a 3R resistor.
Since these are both linear regulators, i.e. they work as variable resistors and "burn off" the excess voltage, you ought to match the required voltage as closely as possible, or else you'll be wasting a lot of power. Also, the transistor acting as the variable resistor can get pretty warm.
I once tried driving 6 LEDs in parallel from 4 AA cell source. 6 LEDs in parallel required 120mA at approximately 3.6V. The transistor had to remove 2.4V with 120mA passing through. It was dissipating 288mW of power. A little over half the amount of power that the LEDs were actually consuming. If I had removed 1 cell, I would have only wasted 108mW of power.
BTW, I came across another circuit in a local electronics magazine which works on much the same principle as the Worldtorch circuit in that it linearly regulates the current going to the LED. But it is more complex because it was designed for 48 LEDs and a 12V source. It was designed so there are several banks of LEDs. Each bank has 3 LEDs in series with a transistor, which acts as the variable resistor for that bank. All the transistors are controlled by one other transistor which sets the current per bank.
If you would like to see the circuit, either check out the Silicon Chip magazine website with the keyword "prawning light", or let me know and I'll scan the schematic for you.
