This is interesting.
The 2006 Mac Mini models had Core Solo or Core Duo CPUs.
The 2007 Mac Mini models had Core 2 Duo CPUs.
Other than the CPU itself, the machines were physically identical. In theory, replacing the Core Solo or Core Duo CPU with a Core 2 Duo should make them completely identical... but it doesn't.
The 2006 model is identified as "macmini1,1" aka "Mac-F4208EC8".
The 2007 model is identified as "macmini2,1" aka "Mac-F4208EAA".
This identifier makes the two systems behave very differently.
The most obvious difference is that the latter model is fully supported by Lion, but even before Lion was released, there was an even more significant difference...
The 2006 model was limited to 2GB RAM. The 2007 model could take 4GB of RAM although only 3.18GB was usable by the OS because of chipset limitations. If you tried installing 4GB of RAM in the 2006 model, it wouldn't even boot.
Now, a community of tinkerers have been trying to figure out how to make a 2006 model think it's a 2007 model by modifying firmware updates, etc. So far they have been able to get the 2006 models to work with 4GB RAM (again, only 3GB usable). They still haven't fully solved the puzzle, though. While the updated systems support the higher RAM limit and report themselves as "macmini2,1" they still have the "Mac-F4208EC8" identifier showing up which means Lion still doesn't work without the install hacks.
They're still working on it, though. The forum thread is 7 pages long so far and is really interesting:
http://forum.netkas.org/index.php/topic,874.0.html