Anyone care to take a guess about what dave's current project is that required the DC-DC converter? The only thing that I can think of that specifically needs 5V/500mA is USB. Assuming then that it is USB, and that it still needs its own power supply at those specs (not drawing power from the lines) makes me think that it is either:
1: A USB device charger (ala mintyboost, but smaller)
2: An embedded USB host, possibly for a small sensor/demo board. I'm leaning toward sensor because demo boards can usually be powered externally. Further, a sensor board's USB host could be used to, say, download data onto a text file on a USB memory stick. The one flaw I see in that is the sheer absurdity of embedding a whole USB memory stick onto a board when a simple SD card would do.
On the other hand, maybe I lost the track completely when I thought "USB." Any other thoughts/suggestions?