I'm running the 2007 Aluminium iMac with the ATi HD2600 and I'm having similar problems.
If I hit CMD-Return or CMD-Tab out it goes to the desktop fine but when I try to bring CoH back up it goes black. I also need to quit to get it to run again.
Very annoying. Not sure if it's Apple's side that needs fixed or an update to the client, hope someone looks into this.