Java Web Start does not come with 64-bit builds of the JDK. Why is this? What is lacking that keeps it from building and working?
Apparently, there is no reason, since its in JRE6u12. http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4626735
Thought you'd might want to know the new update is out: http://java.sun.com/javase/6/webnotes/6u12.html
64-Bit Browser Support for Java Plugin and Java Webstart This release supports the New Java Plugin and Java Webstart on AMD64 architecture, on Windows platforms. A Java offline installer (JRE and JDK) is provided. Note, if you use 32-bit and 64-bit browsers interchangeably, you will need to install both 32-bit and 64-bit JREs in order to have Java Plug-In for both browsers.
Mostly its a lack of demand. You only really need 64-bit version if you intend to run more than 1200 MB of memory for a web start client. Otherwise it doesn't make much difference.
Do you know of any examples of a web start application which uses this much memory?