I say “was” because Apple now provides a Java 6.0 update. But you know how Apple is: a long time with a (very) few amount of RAM on their computers, low-end graphic cards on their high-end computers, combo drives on their first priced MacBooks, etc. Yes, they sometime take some decisions which are difficult to understand. And oops, they did it again. You want to use Java 6 on your Mac, you'll have to own an Apple computer with a Core 2 Duo CPU. You own a PPC based Mac ? forget it. You own a core duo based Mac (it only two years old) ? forget it to. You really want to use Java 6 SDK ? Buy a new computer… or not.
If you really need to use Java 6 SDK and don't why you need to buy a computer with a 64 bit CPU (Java 6 works perfectly on Windows based computers, regardless their CPU), you have another solution : SoyLatte JDK. SoyLatte is Java 6 port based on FreeBSD Java, which is based on Sun's open-sourced Java. It works on all Intel based macintoshes (sorry for PPC owners).
To use it for development, you just have to download the archive, unpack it and declare the new JAVA_HOME to your development tools.
Oh, you have another solution to get an Apple Java 6 SDK for your computer: you just have to wait, but nobody knows how long…