Two things can have happened
1. the font you changed to is corrupt and that can cause all sorts of problems, including corrupting the font cache.
2. The font is located in a directory that has it permissions set to read only or the font is set to read only you will have to change the permissions manually on a pc.
3. Your font caused your prferences to be corrupt.
4. The settings folder needs to have it permissions repaired.
Maybe tha t is more than two.
I am having the same problem. Every time I close Illustrator it will either close and then a windows error comes up saying "Adobe Illustrator CS5.1 has stopped working" or it will freeze and hang on exit and I have to force close it. Wasted 4 hours with tech support and nothing changed. Tried deleting the pref's file, the entire en_us folder, I reinstalled with a fresh reboot inbetween, etc. Nothing is fixing this, but I assume it's font related.
I think you have to repair the permissions.
Also you say you are trashing the preference the whole en_us folder that is not the right folder as far as I know the folder shoul be called Adobe Illustrator CS# Settings that is the folder you shoud be trashing to start with fresh preferences.
What system are you running?
You can do a search for finding the Adobe Illustrator CS# Settings folder for your system.
BTW ypou have to qui5t Illustrator before you trash or rename the folder then after you trash ir rename restart Illustrator.
But I think you have too repair the permissions.