OS X 10.5 on macmini3,1 model? To reproduce/debug an issue exclusive to OS X 10.5, i was trying install Leopard on my Mac Mini (model "macmini3,1", came with Snow Leopard).
However, when trying to boot from a Leopard install medium (tried both from DVD and USB stick) the machine just hangs:


when manually choosing the boot medium: hangs with the boot medium selection screen still displayed 
when running the install app from Snow Leopard and letting it restart the machine: hangs with the bright grayish screen and no logo or progress indicator


Does anyone happen to know what could cause this / how to work around it?  

Or could my models hardware be too new for 10.5? Although a web search suggests there were/are macmini3,1s with 10.5 running. Also, this question and it's resource suggest it should be working.
The usual Apple way is that a set of generic OSX disks only support all machines that come out before that OS (How would any know what a new machine might have changed). Machines released after an OS release will include boot disks that can be loaded by that machine but earlier versions might not.

Thus I suspect that your mini is unsupported for Leopard or people bought it earlier with a leopard version for that machine.