Personally, I think this post by Daring Fireball author, Mr. Gruber, sums up the lack of Java 6 on OS X 10.5 fairly well.

Daring Fireball: Shipping Means Prioritizing:
The only way to ship software is to prioritize, and prioritizing means dropping things that are less essential in exchange for things that are more essential. Obviously, for Apple, Java 6 is not a priority. And, judging by reports that even Java 5 support is worse on Leopard than it was on Tiger, Java as a whole is not a priority for Apple.

Poor Java support on Apple is nothing new. Java 2 never made its way to OS 9 and earlier, though let me tell you, the members of Apple's java-dev list were clammering for it like mad. (And once they got it, the complaint was, "Where's my Java3D NOW?!!!")

What people seem to miss is that, unlike Windows and Linux, Apple is fielding Java in-house. Sun isn't creating Apple's VM; Apple is. Ask Microsoft what priority Java is for them. They'll tell you it isn't on the map. Linux? Better, and the Blackdown Java port was awfully impressive. You could get Java 2 on Mac hardware more quickly by installing Linux on your PowerPC than waiting for Apple's Java 2 in OS X. Still, I'd imagine most Linux-ites hacking Java use Sun's now.

That Apple does Java at all is impressive. That every OS X box has Java 1.3+ out of the box is great news. That Apple has given Java all sorts of platform-specific extensions and then broken them with reckless abandon is a bit of a pain, but taken together just make another reason that any Java developer should have seen 10.5's poor support coming.

Look, if you've got Apple hardware and still want to look k3wl at JavaOne, create a partition and dual-boot Linux. Whining over.