I have long heard that the recent Macs would boot from external USB DVD drive. Since I have an external Plextor PX-708UF, I have the luxury to try it out now.
I spent a great deal of time on Thursday and Friday night, but I couldn't manage the Mac Mini to boot from the USB external drive. I resorted to rEFIt, but it wasn't successful either.
Turns out that I have to connect the USB to my Mac Mini's onboard port, rather than any external USB hub.