It's only specially-prepared "hybrid" ISOs that you can dd straight to a USB stick. The older method was to have an ISO for burning to CDs, and a different kind of disk image that got dd'd to a USB stick. My distro (Arch Linux) is just now transitioning over to using hybrid ISOs.
Also, I'm assuming this isn't a general question about how to create a bootable USB key. The questioner specifically asks about Macs. I don't know how often that will be possible. I've made bootable USB keys that work fine in all my non-Mac computers, but when I try to boot my Intel Mac Mini from it, here's what happens. I have reFit installed on the Mac Mini's main drive, this will show a graphic at bootup which shows all the available drives or CDs I might boot from. I think it also shows the USB if that's plugged in. However, it's not possible to boot from the USB. I get a message saying that my Mac can't boot from an external USB drive, only from Firewire drives.
I don't know if it's possible to work around this with this hardware. From the link another answer provides, it looks like other Mac hardware does permit booting from USB drives.
What Distro? Ubuntu offers functionality to create USB-Boot-Sticks. – Bobby – 2010-04-02T12:10:23.453
I'd prefer a solution which would work with almost any iso file. If this is not possible then at least Fedora and Ubuntu should be ok. – lhahne – 2010-04-03T04:56:37.197