2009 Mac Mini HDD as an External Drive for a 2011 Mac Mini HDD I have a unique plan:

I have a 2009 Mac Mini mid-2009 with 10.6 OS X.  I just got the 2011 Mac Mini with 10.7.  But I am planning to use the hard drive from the 2009 Mac Mini with 10.6 OS X as an external USB drive for my new 2011 Mac Mini.  

Question:  Can I boot from the external USB with 10.6 from my new 2011 Mac Mini?  (Using the Option key when turn on the computer, etc.)  So, that I can boot into either 10.6 (on the external USB drive) or 10.7 (the internal drive) from my mini 2011?
Intel Macs can boot to a USB volume, but in this case it will not work. The reason is because the new Mid 2011 macs ship with 10.7, as you mentioned. Macs ship with a minimum supported OS, and will not boot or allow installation of any OS prior to that. Therefore, a Mac that shipped with 10.7 will not boot to a 10.6 install. You'll probably be able to see in when holding option at startup, but if you select it it will likely show an Apple logo and hang there.