In Leopard, Java 6 is 64-bit only. That means
1. You can't use it for browser applets (since Safari and all other browsers in Leopard, are 32-bit)
2. You can't use it at all on G4 or Core Duo systems (since those processors are 32-bit)

In practice, this makes Java 6 pretty useless in Leopard for everyone except people who run servers, and Java programmers.