I messed around some more with the romanblack circuit and I have to comment some things.

1) The output voltage that this circuit produces is dependent on the zener voltage and the Vbe of the lower transistor. The Vbe and zener voltage both will change with temperature. The zener voltage will be heavily dependent on the bias current supplied to it by the resistor that is tied off to the +Vin.

2) The output regulation of this supply at a given temp and bias level is not too bad for a static load. If the load has a dynamic characteristic the regulation suffers greatly. This is expecially the case when the transient effect of the load tries to source current back into the output stage of the power supply.

Conclusion:
If you are going to build a buck regulator of this type then I really suggest that you look at the small chips from National Semiconductor. They will at least give you switching times that are higher which leads to smaller inductors. The comercial switcher parts also offer far far better references than a zener diode based via a resistor.

Michael Karas