I know I'm a late adopter and all, but I thought I'd provide some information on Eclipse on OS X Leopard. First off: it runs fine. Many of the concerns were that 64-bit Java would break Eclipse because of the SWT/Carbon underpinnings, which are 32-bit, as I understand it.
Well, rest easy - as it does with many new technologies, Apple has left full backwards compatibility in place. It appears that Java runs in 32-bit mode by default, so basically everything that worked previously (including SWT) will continue to work.
So what about Eclipse on 64-bit Java? Yeah, it doesn't work. So - to all those that predicted this, kudos. You got it right. Adding "-d64" is the flag that tells the JVM to run in 64-bit mode - and adding this to eclipse.ini "-vmargs" causes an exception when SWT is loaded.
Want to test if you can run 64-bit Java? Here's what to do. In Terminal, run the following command:
/System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Commands/java -d64 -Xmx2560M -version
Running this should output something like so:
java version "1.5.0_13"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_13-b05-237)
Java HotSpot(TM) 64-Bit Server VM (build 1.5.0_13-119, mixed mode)
If you don't get this, or get some type of message like "Cannot run Java in 64 bit mode. Continuing in 32 bit mode." then you may not have a 64-bit chipset. Notably, the PPC chips (while G5s are 64-bit) cannot run 64-bit Java). Further, the first generation of Intel chips released by Apple, Duo Core, are not 64-bit. Only the Core 2 Duo (and later) chips are 64-bit. Finally - if you ran the "-d64" test by just specifying "java" on the command line, instead of the full path - you may be running into a bug that was discovered too late to be fixed in GM.
I'll give a shot at running 64-bit Java from within Eclipse (say, to run JBoss AS - that should work). I'll report back with results!