Mac OS 10.5.6 thwarts jailbreakers (Updated 3x)
Unfortunately, it has also unleashed its share of headaches.
iPhone Alley notes that 10.5.6 prevents a Mac from recognizing an iPhone in DFU (Device Firmware Upgrade) mode. DFU mode bypasses the current OS installed and allows you to upgrade or downgrade your OS. This makes it impossible to jailbreak and/or unlock an iPhone using PwnageTool or QuickPwn.
I recommend waiting at least 72 hours before installing any update to the Mac OS, even longer on a production machine that you use to make a living. It's best to wait several days and read the troubleshooting sites and the Mac OS X Leopard thread on Apple's discussion forums before diving in. After you feel comfortable that all the bugs have been worked out, make a bootable backup of your system (and test it!) before pressing the install button.
Have you installed 10.5.6 yet? Any problems?
Update: The Energy Saver presets previously available under the battery icon in the menubar seem to have been removed in Mac OS X 10.5.6 (picture).
Update 2: Macworld's Rib Griffiths took notes that Apple apparently updated various Bluetooth software components, Migration Assistant got some polish, and the Podcast Capture app was updated to version 1.0.3. Other low-level drivers, frameworks, and extensions were also updated, and even the Apache Web Server manual got some attention.
Update 3: New threads in Apple's discussion boards complain about everything from broken AirPort, support for specific Bluetooth devices getting removed, and choking installations (TUAW).