A Unity ID allows you to buy and/or subscribe to Unity products and services, shop in the Asset Store and participate in the Unity community.
Separate names with a comma.
Just move your existing Unity folder to other location or rename it. E.g. "Unity 3.5"
I have a similar problem. With Unity 4.0 and 4.1. However I'm not seeing any spinning pinwheels. CPU and memory load are far less than 100%....
Problem still exists 4.0.0f5 on OS X 10.8.2