It is not the case that G4 or G5 CPU's are insufficient, several users on the macrumors forums have managed to get some of the older developer builds to work with the retail release of Leopard / 10.5 on G4 and G5 cpu's, a user on that forum had shared it on the torrent network and was going to upload it to rapidshare however he was told not to by the administrators, regretably the torrent gets stuck at 12%.
It really sucks being deprived of features that we payed for, this is one of those situations where apple is trying to get people to upgrade to gain features they should rightfully have the ability to use on their current hardware, regardless if it runs slow or not they should have given us the choice.