Well, according to DS's list, the highest any integer-numbered OS has gone is .2 before jumping up to .5 (except 4.3). If they keep up the trend, then, Panther should officially be called Mac OS X 10.5 because they almost never have had a .3 and never a .4 version.
Anyway, that wasn't my original point. Originally, I was saying what cat will be after Panther? If Apple continues to use large cats, they'd have to take their pick from Lion, Mountain Lion, Lynx, Leopard, or Tiger... unless I'm forgetting something.
Wouldn't you want to buy Mac OS X 10.6: Tiger?
On a side note, that list sure makes Windows' scheme (for a while) of numbering after the year really simple... too bad they stopped. It would be easy to tell both the version and the age of an OS if they named them after the year... like Mac OS 95, Mac OS 2000, this summer Mac OS 2003... They should label their hardware like that too.