Sorry for the slow reply, I missed this one ... (and this isn't my day job :-)
First of all, for the benefit of anyone else reading the thread:
the existing 32 bit x86 Java implementations will run on AMD64/Opteron.
Both SUN Microsystems and IBM have announced their intention to release AMD64 implementations - go to google and try typing AMD Java 64bit.
Here is a link to the Sun Microsystems "Java on AMD64" press release
Sun's press release states the support will be for Windows and Linux. Given the 'rumours' of a Sun server using AMD64, it's may be on Solaris too.
I think 64 bit Java was released late 2000, early 2001. I haven't used that implementation, but it is described, if memory serves, at Sun and maybe IBM's developer sites.
[Go to google and try "site:java.sun.com Java 64 bit" etc]
Good enough? Keeping asking if I've missed something.