Check out Sparkfun..they've got a breadboard PSU with a switch for 3.3v and 5v. I've implemented one on a simple AVR prototboard, that just uses a 2-pin header for a jumper. Pretty accurate. But it used 4 resistors as the jumper 'shorted' a resistor in parallel with another. 240 ohms between out and adjust (two 120's in series) with a 390 and 330 in series from adjust to GND. The jumper just shorts the 330 ohms.
probably an over complicated way of doing it, but I built it without designing it first, just a 15 minute job with the parts I had. Roughly 4.97V and 3.29V.
Blueteeth