Apple has a developers' preview version JSE 6, but they don't yet support it. Hence, the Mac version of the Java SAPgui doesn't yet use JSE 6, even though the Java SAPgui on other platforms does use JSE 6. It is likely that by the time of SAP's next major release of the Java SAPgui, Apple will support JSE 6 and the Mac SAPgui will require it.
Currently Apple's implementation of JSE 6 requires a 64-bit Intel processor running Leopard. If this remains true in Apple's final release of JSE 6, then the next major release of the Mac SAPgui won't run on any MacOS X prior to Leopard and won't run on PowerPC or 32-bit Intel based Macs even with Leopard.
I'm told that Sun has both 32-bit and 64-bit JSE 6 JVMs for Windows, for Linux, and for Solaris.
My organization (MIT) has submitted a bug report to Apple about this. It is possible that Apple might pay more attention if a number of their customers made a joint request.
If your organization would be interested in joining such a joint request to Apple to support JSE 6 on (at least) all supported Leopard platforms, please let me know.
/David Rosenberg <email@example.com>
P.S. If you work for Apple and you think you might be able to help with this, please contact me.