We still don't think it's worth the time and effort spent porting it at the minute - at least not until we have a version 1.0; by the time Palringo goes out of Alpha every Mac user will have updated to either Leopard or Snow Leopard *shrugs*.
If there's time and enough demand, we'll consider it.
No hard rule however, so far only a dozen people have requested a Tiger back-port, which is obviously not enough.