EA has a notice about the new patch and you can see the information here. This page also provides a link to the manual download.
This is the same download that I received from another site as I also downloaded this and saved it to my desktop to compare them.
Edited to add: EA now has added Mac-specific support to its support page, cited by mtglady above. Yay!
Pallyndrome, if you live in Australia and bought your base game on a DVD, it looks as though you should use the European patch.
You should not need to uninstall anything. Simply be sure to select the base game itself, not The Sims3 folder or any of the expansion pack files, when you are asked to select and patch the Sims3.app.
Some people had problems unarchiving the .tgz file, and some had problems that apparently resulted when they selected The Sims3 folder instead of the actual base game application, but it worked seamlessly for me. I hope you have the same luck.
There are some interesting new features in the patch/update that I thought would be in Generations, not in an update to the base game. You can select the length of time for any stage of life now, and every little achievement prompts a "memory" that can be stored in a scrapbook.
