I'm not talking about altering a few settings here; I'm talking about actually modifying the BIOS itself. Some people have already gone as far as modifying their BIOS splashscreen; lots of motherboard manufacturers ship their boards with a utility that enables this. However, there's a lot more to be done, such as unlocking hidden menus, or updating embedded ROM images. For example, in the past I have updated the integrated Silicon Image RAID BIOS on a motherboard of mine.
Of course, messing with your BIOS isn't to be done on a whim. If you do it wrong, you can cause yourself some serious headaches. However, if you DO need to do some 'work' on your BIOS, then you may want to check out the latest version of Borg Number One's BIOS-Tool-Collection. There are lots of tools for modifying a variety of BIOS elements. Check it out here.