The tl;dr version is "While 10.7.5 and 10.8.2 brought this problem to light, the problem is actually on the EA end (actually one of their contractors/suppliers) and you should absolutely be contacting them for support with this issue." I propose a workaround at the end of this message.
I believe this is related to the problems reported by Wineskin and Crossover and other utilities for running Windows programs on Mac OS X. While Wineskin has released an updated version that fixes the bug, EA has yet to do so for Sims 3. And to be honest, I would not wait with bated breath.
Let me explain.
Many games that run on OS X use "Wrappers". Basically this means they take the Windows game, and "Wrap" it in another application. This wrapper notices when the Windows application tries to do something the Windows way, and translates it to the Mac way of doing things. Everything from playing sound to drawing a single pixel on your screen is translated completely behind the scenes. These wrappers almost all use technology from the Wine project, which for a long time has worked on allowing people to run Windows application on other operating systems.
Sims 3, and many other popular games, use a wrapper called Cider, from Transgaming. If you Ctrl+click on the Sims 3 application and Show Package Contents, and then select Contents - Resources, you'll see a transgaming folder. Inside there, you will see a folder called c_drive, and if you poke around further you can still see the Windows .exe files.
This allows game developers to develop only one version of the game (for Windows), and then rely on this wrapper to also sell the game to Mac users, instead of developing a custom version of the game.
In the 10.7.5 release, Mac changed something (I don't know what) that brought to light a bug in XQuartz, a component important to running Windows applications on a Mac. This package is used by Wineskin and possibly Cider as well.
The process moving forward for EA is wait for transgaming to update their wrapper, and then package that wrapper in an update for Sims 3. If it's a priority, it's a week or two of work. Definitely contact them to ask for support; while the update from Apple brought the problem to light, the issue lies with EA (specifically with Cider, which I assume uses XQuartz).
I employed a workaround, both to continue my play and to verify that the problem is with Cider: I ran Sims 3 in another wrapper. Because the Sims 3 application is not affected, you can actually replace the wrapper it ships with (Cider) with another wrapper (Wineskin) fairly easily. I cannot comment on what this might mean in terms of the EULA, but:
- create a Wineskin wrapper
- copy the contents of the c_drive in Sims 3 to drive_c in your new Wineskin; overwrite if prompted
- copy the contents of Frameworks in Sims 3 to your new Wineskin, overwrite if prompted
- copy the Preferences folder over
- run the appropriate registry update files to complete the Sims 3 install
Sorry that is not a very accessible todo, but honestly if that is all Greek to you, you should focus your efforts on bugging EA or restoring your Mac from an old backup. You can also buy the Windows version and install it to a Wineskin; THAT there are instructions for online.
Hope that helps without overwhelming!