Where's the BIOS on an Intel Mac?

When I use my laptop, I have no trouble at all accessing the BIOS and changing things like the boot order.

However I recently got an iMac and have found that not only do the usual keys not get me to the BIOS, but when I find one that gets me to a boot menu (holding option when booting up) it won't recognize my USB boot drive of Ubuntu!

How can I access the BIOS menu like I could in a PC?

There is no BIOS on a Mac. It uses EFI, which can be configured from within OS X e.g. using bless from the command line.

You are able to boot from external USB drives that satisfy some requirements.

There are some key combinations that change the boot sequence, but there is no real boot menu.

Change the permanent preferred boot device in System Preferences on OS X or Boot Camp settings on Windows.

If you want more than what's available out of the box, you need to e.g. use rEFIt.