I currently have two Mac Minis, one is the current gen, another is a 2009 model both loaded with OS X Lion. I am trying to boot the new Mac Mini with the hard drive pulled from the old one using a SATA to USB adapter. When booting, I pressed and held the Alt key. When the old hard drive appears as an external drive, I choose that drive to boot the Mac. That's when the prohibitory sign appears on the screen.
The current-generation Mac Minis originally shipped with OS X version 10.8.1 (build #12B2080), and will not be able to boot any older version of OS X.
I'm pretty sure the generic build of 10.8.2 will work on them, so one possibility would be to upgrade from Lion to the latest Mountain Lion. Depending on what you're trying to accomplish, another possibility would be to use the Migration Assistant to copy your account/documents/whatever to the newer Mini, as described in "Migrating a Time Machine backup to a new Mac" in Apple's Mac Basics: Time Machine.
See also Apple's Don't install a version of Mac OS X earlier than what came with your Mac.
