It's a great idea, esp. as it would also let one Arduino board burn a bootloader onto another.
I know Nick Zambetti has been working on exactly that, but it's not quite ready yet. He's found a library to use a ATmega8 as an STK500, but it needs some slight modification to work with the Arduino board (e.g. it's designed for a 4MHz clock).
Maybe you'd be interested in helping out/beta testing this? If so, just post here or email me or Nick.