Well, I just successfully booted my Mini 5,2 from the DVD amd64 iso image burnt on a DVD using my cheap Samsung external USB writer and apple's Disk Utility! No need for the expensive Apple USB drive! Booting is capricious however, I.e. sometimes Ubuntu boots sometimes it's just a black screen (must be a grub issue on ubuntu's side of things). Though it booted the first time (after I chose the penguin icon with a dvd showing in the rEFIt boot menu), on other tries it did not. The trick I found that made if boot was to choose the same icon but have a USB key with FreeBSD 9.0 installed on that key inserted in one if the usb slots of the Mini. Bizarre but true. Anyway, happily booting Ubuntu now on my Mini 2011, yeah!I recall Ubuntu still does not boot that way.
Maximini