I needed to run a few simple tests booted to Knoppix on my Mac Mini but the Mini refused to boot to the cd drive, despite me holding down the "c" key during boot. I resorted to using the Startup Disc applet in the System Preferences and finally I was able to boot to Knoppix.
I ran my tests in Knoppix and rebooted, ejecting the Knoppix cdrom. I expected the Mini to boot into OS X since that was the only boot volume available.
Guess what? The screen turns black and there's a message saying there is "no bootable device." Yay!
I scurried off to google and the Apple formus to find a solution. I tried every key combination they suggested and none worked. After wasting an hour trying everything short of defenestrating the Mini, I stumbled over a post suggesting my compact Apple keyboard was blame. Turns out these compact, USB aluminum keyboards Apple makes are incapable of producing the signals required to do important stuff like access the boot manager, reset the nvram etc.
In short, they are not real Apple keyboards.
I had to attach a full-sized, Apple keyboard to invoke the boot manager. What bullshit!
My thanks to Apple for charging me $50 for a crippled keyboard.