Just about everything has been running great with CS5 on my Mac system until this afternoon. When I started the application, it would begin loading all of its resources and then just quit and give up. I had just installed a plug-in before the flakiness, so by simply removing the plug-in, everything went back to normal. Here is what happened in case you encounter the same problem:
AE CS5 comes with a new plug-in, Digital Effects Free Form. If you register with Digital Effects, you get a free plug-in, Falloff Lighting. I installed the plug-in in the Adobe/Common/Mediacore folder. It seems Premiere Pro tried to load it, was not at all happy about it, and called it a day.
Maybe it was a 32 bit plug-in?
Yes it is 32-bit