I've been trying to do this since yesterday, following the instructions given on ubuntu.com's official page here:
I can complete every step without error. However, when restarting the mac, the flash drive simply reports 'missing operating system, press any key…'. From there, there's nothing I can do except a hard reset.
I've tried Unetbootin, but that won't create a usb that's bootable from a mac.
edit: I've also fixed the partition tables with rEFIt, but still no joy.