The LEDs have a forward voltage drop of 3.2V (quite high!), so I can just hook 'em up directly to the batteries. However, rechargables have a maximum voltage of 1.2V, and at 2.4V, the LEDs just don't get that bright...
I had seen a voltage regulator circuit based on the LM317 somewhere, and I wondered whether it could be used to boost the voltage -- but alas, that is not the case.
But the LM2621 from National Semiconductor and the MAX1674 from Maxim can!
I've registered and ordered samples of both -- power supplies isn't my forte, but if I get this to work, I can suddenly create battery-operated stuff with relative ease as well!