I'm glad to know it's possible, but I'm still not getting it.
Here's what I've done:
Uploaded the sketch Examples->Arduino as ISP to my Diecimila board
Chose Tools->Board->Arduino Uno (I'm using a 328 on my breadboard so I figured any of these would work)
Chose Tools->Programmer->Arduino as ISP
I can upload sketches to my breadboard, but I don't have any option to burn the bootloader to it.