If you're willing to read through the gazillion posts to Apple's
java-dev list on this subject, you can absorb a lot.
But Apple just put out a tech note on 'resolved issues', but there is
at least one tech note to come in the near future.  Some people have
speculated that 1.6 for Leopard will be out, at least in Preview form
soon.

http://developer.apple.com/releasenotes/Java/JavaLeopardRN/index.html

Also, on Leopard running 1.5 you can't invoke the 64-bit mode (-d64)
on anything but a Core 2 Duo.

On 10/31/07, Miles T. Parker <[log in to unmask]> wrote:
> Argh..I just ordered it! Couldn't find a whole lot about this issue
> online, except that 1.6 isn't included, which doesn't surprise me at
> all. I'm wondering if this mainly impacts visualization or if it is
> GUI in general (I don't care so much about 1.6 improvements to Swing
> as I'm spending much of my time in Eclipse these days..) Or..eh..if
> it is Java performance altogether, that would really suck. I think
> I'll install it on my wife's MacBook first..
>
> On Oct 30, 2007, at 9:56 PM, Sean Luke wrote:
>
> > You've probably heard: OS X 10.5 (Leopard) runs Java exceptionally
> > slowly if at all.  This is because they've turned off OpenGL
> > hardware acceleration (or what little of it they had in 10.4).
> > I've been told that a number of Java simulation environments, such
> > as Processing, are having difficulty.  I have not tested MASON
> > under Leopard and cannot verify that it will run well or not.  But
> > given Leopard's serious Java problems, you may wish delaying any
> > upgrade until a later version (10.5.1, perhaps).
> >
> > Sean
>