First of all, I fully expect that Apple will release an update to Leopard that will include Java 6 before the end of the year. But it doesn't matter too much because Java 6 was mostly a performance release for Sun. There's some nice things in the Swing implementation included on Hot Spot. Neither one of these things is even relevant for Apple. There are some language features (debugging, StAX parser) but these are pretty minor.
So to me it doesn't matter too much that there is no Java 6 for the Mac. Oh, but there actually is, or was. I personally had some problems with it, so I wasn't too surprised that it's no longer available from Apple.
Things only become problematic if Apple takes a long time to support Java 7 when it comes out, especially if you assume there will be a lot of language changes in Java 7. If that happens, then it could be conceivable that developers won't be able to use OSX. Others have pointed out that a major litmus is Eclipse. If Eclipse will run on OSX, then all is well. Eclipse 3.3 was released just a few months ago and was the first version of Eclipse to require Java 5... Eclipse is obviously important for Java developers, but also for Flex developers and even PHP developers.