That's interesting! I have a first gen MacPro- Intel. I absolutely detest Apple doing this.  It is a trick to try and get people to buy a new computer.

A few years previously, I was running a first gen G3 with a G4 processor upgrade. Apple decided that OS 10.3 would NOT install on a computer without a built in USB. USB card.

Someone made a hack that fooled the intaller. As a result, I was able to install both 10.3 and 10.4-which ran perfectly well.

 

Apple has made it real easy for me - Lion won't install on a boot disc that is a raid volume and ML won't run on a 1st or 2nd gen MacPro unless I hackintosh the bootloader.  I can easily do it, but I really don't need all of the iToy features on my desktop.

So it looks like I'll be on 10.6.8 for a while.