It was really only with the release of 10.2 that Apple started to really emphasise the feline code names of the OS. Jaguar had the animal skin "X," Panther brought a bit of maturity to the OS with the brushed metal and Tiger was the first to be marketed almost exclusively using the cat code name.
I really have high hopes for 10.5. Tiger was really a massive "behind the scenes" overhaul. Most of the work was in the code with but a few new visible features. Hopefully Leopard will be able to make full use of that code and really take the fight to MS's Vista (which WILL be a good OS, don't go kidding yourselves, this will be a tough fight) and really bring the Mac platform into the public eye.