I don't see why Leopard wouldn't work. I've had no issues one any of our Macs at home upgrading from Tiger to Leopard. At work (I'm an Apple technician) I've seen quite a bit of random issues that usually result from upgrading for whatever reason. I haven't run into too many applications that don't work without an upgrade after Tiger (Parallels and Quickbooks are two that come to mind but they have updates for Leopard released now).
Tiger is more light weight and overall feels more responsive that Leopard so far though. I am thinking 10.5.2 should fix that quite a bit (it's going to be the largest .x release update Apple has ever done).
I will be running Leopard on my Mini when I get it, partly because of Front Row. It's actually not a bad Front End for a Car PC.