I wouldn't class Java as a first class citizen on the Mac; Java is a first class citizen for Sun.
Apple simply do not and will not offer the same driving force as Sun (or the open source community). IIRC, Java latest versions seem tied to latest OS X versions (hence no public Java 6 on 10.4 or Java 5 on 10.3). The best development tools are the same as on Windows, Linux and UNIX i.e. Eclipse or NetBeans, not Apple ones. Look what Apple actually bundle and provide in a host of other languages, such as Python or PHP. Present, yes, but well out of date. The odd article here and there on the developer network and again, often out of date.
IMHO, Apple's heart is, and always will be, with their own native toolkit(s) and they will do as much as they need, but no more, to attract developers using other platforms (the same can be said of MS but the size of the Windows platform means the impetus again comes from 3rd parties who will often not make the same effort for the Mac, *cough* *cough* IBM). Stuff that Apple no longer deem profitable
will be deprecated and
may be turned over to the community.
I suggest you subscribe or browse the Apple-Java-Dev mailing list - my thoughts were captured quite nicely by this recent post
The smart money is that 1.6 is waiting on Leopard - i.e. less than one month if you're brave enough to handle the knuckle-whitening terror of any 10.x.0 release. Who knows, perhaps Leopard will be different...?
Of course, officially this is all speculation. After all, part of the fun of being an Apple developer is being treated worse than any other community we know of and still enjoying it. Like being the willing love-slave of a beautiful, seductive yet cruelly sadistic goddess.
It's the Apple condition.