Xcode 4 and Snow Leopard
If you have registered for the paid iOS developer program, you should be able to see Xcode 4.2 for Snow Leopard available in the iOS Dev Center.

Here's what I see from a paid account: 



Here's what I see from a non-paid account (there's no Xcode 4.x for Snow Leopard):



This also explains why Xcode is a paid download in the Snow Leopard App Store, but is free in the Lion App Store.