So Apple finally brought the Java 6 Preview back and made it available to the Apple Developer Community via their ADC program. However, there are restrictions:

The Java SE 6 Developer Preview 8 requires

• A 64-bit capable Intel-based Mac, meaning your Mac needs to have either a Intel Core 2 Duo or Intel Xeon processor.
• Mac OS X v10.5.1 or later installed

For Intel 32-bit processors, like Intel Core-Sole or Core Due, that aren't supported by Apple's Java-6 Preview there is still a Java 6 Port for Mac OS X 10.4 and 10.5 available: SoyLatte
PowerPC based Macs, like iBooks, PowerBooks, and PowerMac G4s and G5s are left completely in the dark, when it comes to Java-6 support.