Looking at the development od Tiger, it doesn't actually seem to be any different from panther other than a few extra features and some added functionality. Yes it is 64 bit, but it's still going to work on the current line of 32 bit G4 processors (eMacs, Powerbooks, ibooks, Mac mini) so for that reason, I don't see why there would be any problems at all running Panther apps on Tiger, they just won't take full advantage of the new OS.