No, I have a MacAlly usb/firewire enclosure for both laptop and desktop hard drives...
so I can just boot off USB on my Mac Mini and off a Mac Book Air with usb?
I had no idea. I guess you could NOT do this with the old aluminum MacBook?
Confused over this whole USB booting issue. I thought it was only possible to boot into Windoze or something.
If you have a valid boot drive, then yes, you can boot off of USB. All Intel Macs are capable of booting from USB. PPC Macs cannot boot from USB, but they can boot from Firewire.
You could install OS X to an external USB drive (or even a large enough flash drive) and boot to that installation. It's great as a diagnostic tool. You could also clone your installation media to a USB flash drive and use that for installs. The benefit of that is that it would be much faster than installing from a DVD.
In the past, Windows has had some issues with booting from USB or Firewire because the USB/FW drivers are not loaded until later on in the boot sequence. I'm not sure if this is the case with Windows 7, but I know it was for XP and Vista. If you can follow directions, there are ways to create a custom installation that will let you boot Windows from external drives, but in general it doesn't work.