Bootcamp on my Mac says its only for windows dual boot.
Then you are probably right. As I said, I know very little about it. Apple, of course, has a history of artificially limiting functionality for marketing purposes. Still, I imagine some enterprising hacker will figure out a way to make it boot Linux (one of these days...)
Anyway, as the previous poster indicated, there are always alternatives (e.g., Rod's EFI thingie).
How does VirtualBox work? Is there a way for run Linux from a USB HDD? My MacBook Air only has 128gb to start with and I have a 500GB USB HDD sitting here with nothing on it.
Yes, you should be able to put the "virtual disk" file anywhere you want. That should work fine.
And some folks need to stop being fanboys and see the forest behind the trees.
(One of the best lines I've seen on this board lately)