AFAIK, No and yes.
Short answer: No, you cannot run a USB Linux Distro by just installing it to a USB drive and rebooting your Mac, but you can run a Linux distro off USB when you use a bootloader on the internal drive or on a CD/DVD to boot the USB drive.
Long answer: The EFI on Intel Mac's refuses to boot off USB what it considers a legacy OS, i.e. Windows & Linux. You can bypass this issue and run Linux off a USB drive by either first running a Linux Boot CD that will allow you to boot a linux image on a USB drive (I've done this with a Backtrack CD that contained LILO to boot a copy of Backtrack on a USB flash card) or you can install a boot loader on a partition in the internal drive so you can boot the USB drive (I've done this with GRUB to boot a copy of Backtrack on my iPod). If you choose to install a bootloader on the internal drive, do a backup first and be very careful before you start partitioning/formating the drive
Also keep in mind that you have to install the bootloader to the partition in which it will reside in,
not to the MBR and you'll also need to install rEFIt if you do a bootloader install and you want to jump between Windows and Linux without chainloading.