Ok, it seems like I have become a FCP idiot. WE updated to Universal and sometimes i'm getting thie green background on unrendered footage, but the render options won't let me render the footage?
I've experienced this when the scratch disk that was selected for creating temporary render cache files was incorrectly set, or set to a directory that I had no permission to write to.
Try resetting your scratch disk, then restarting the program.
If that fails, trash the preferences and when you launch again, select a specific folder to use as your scratch disk (one that you know you have permission to write to).