If you are using Mac OS X 10.5 a.k.a. “Leopard” and run Software Update from the Apple menu you should see that the 10.5.6 update is available now! The knowledge base article describing it is here:
and the combo-updater (weighing in at 668MB) can be downloaded from here:
I’m just downloading it myself so no word on potential problems just yet. I’ll update this post with any pertinent details regarding the update process as needed.
UPDATE: WORKAROUND FOR POTENTIAL UPGRADE PROBLEM
Apple has acknowledged that some people have been seeing a problem when attempting to up grade to Mac OS X 10.5.6. According to this Tech Note the symptoms are:
When attempting to update software using Software Update in Mac OS X 10.5, the update process may stop responding while “Configuring installation” is visible in the update window.The same tech note also gives a fix that requires deleting potentially partially downloaded updates via the Finder.