In particular, if you have a MacBook Air, which doesn't have an optical drive, then the forums are filled with posts of trying the dd-to-a-partition trick and failing. This doesn't always work, even on the same hardware. The basic method is explained nicely here. One method that has worked for a number of people is to dd the bootable USB key to its own partition on your hard drive and then boot off that partition. Longer answer: (Ok, I lied above.) You can, but 'it's complicated'.

(See this for an explanation of the difference)

If you have a newer Mac (64 bit), just remember to use the Mac iso( amd64+mac), not the regular amd64 iso.

If your Mac has a working optical drive, use it. Apple doesn't want you to boot an OS other than OS X off USB.