Update 10/21/2012: This problems seem to be still pretty common. This page gets several hundred page views a day. As such, some of the comments are extremely useful to look at, particularly one of the first ones from Steve. Best of luck! Ask questions in the comments and I'll try to be of help.
This is a pretty scary problem if you run into it. According to the guy at the Apple Store, it's a pretty rare one too. Being such a rare problem, it's not very well documented. It just showed up one time when I rebooted the computer. More or less, when your computer restarts, it's goes to white screen for a while and then it goes to a black screen with the white text "No bootable device -- insert boot disk and press any key" and hangs there. Restart the computer and it does the same thing. Needless to say, when you need to take an online quiz before midnight, you start freaking out.
First of all, you will only have this problem if you had BootCamp running another OS on your computer at some point. If you haven't, then this is not your problem. The issue is that BootCamp did not unpartition your Windows disk space correctly when you removed it. It happened to me because I had Windows 7 on my computer via Boot Camp, but then I uninstalled it and then put it on via Parallels (good choice, by the way.)
After some consulting with my owners' manual that night of the quiz, I found if you hold option (the alt key) during the white screen, it will allow you to select your Mac OS X partition and start your computer as per normal. You can actually keep doing this every time you start up if you prefer. I actually thought it would kind of be a nice security feature because if your computer restarted, no one would be able to figure out how to start your computer.
As I imagine most of you want to fix it, here's how you do it. Open Boot Camp Assistant from Applications/Utilities. Going through Boot Camp Assistant, create another 5 GB partition as if you were going to install Windows again. After the partition is created, use Boot Camp Assistant and delete the partition. If you do it correctly, it should fix your problem.
Your alternative is just to go the Apple Genius Bar and let them do it. That's how I did it because I couldn't find anything on the Internet that showed me how to do it.