How did I miss this? Aspyr released a 64-bit version of SimCity a couple of weeks ago on the Mac App Store, happy days!
Just open the App Store and search for SimCity. Alternatively, here’s the direct link.
The upgrade is free if you have already purchased the game through the App Store, otherwise you will need to pay for it, the price is currently £19.99 in the UK.
Still no word from EA yet on whether they will updating the version available in Origin. To be honest the Aspyr version has always run better and been more stable in my experience so I would definitely go with that option if you want to play. Enjoy!