Although, if you are the millions who owns iPhones that had been jailbroken and/or Unlocked, you need to stay AWAY from this Mac OS X update for now. It has been reported that this version of Mac OS X will not recognize hacked iPhones. Until either this report has been independently verified or until the iPhoneDev Team has a method to get around the problem, hacked iPhone users should stay avoid updating their Macintosh for now.
Please stay tuned here, I will update everyone as news develop.
[Update: December 17, 2008]
The iPhoneDev Team has released a note addressing the issue reported regarding the problem with activating the DFU (device firmware update) mode for the iPhone, after the Mac has been upgraded to Mac OS 10.5.6.
The next release of the iPhoneDev Team’s Pwnage Tool is not expected until Christmas Eve (Dec. 31.2008), but there is no mention if this upcoming release will resolve the DFU problem.
[Update: December 19, 2008]
Finally an Automator script that will perform the fix suggested by the iPhoneDev Team. This method will not require a layman to jump into Terminal and fuzz with the inner workings of the Mac.
I have yet to try this so will report when I have.
[Update: December 20, 2008]
Now there are two solutions: one is to connect the iPhone to the Mac via an USB hub. The other is to replace the IOUSBFamily.kext with the version from 10.5. Someone had created an AppleScript to do this for you as explained in the iPhoneWiki forum thread.