The only file I found was Library/Arduino/preferences.txt (I am using a Mac with Lion), and I did not find the line you mentioned.
[Nevermind, I found it here... http://arduino.cc/forum/index.php/topic,58670.0.html ]
I changed the signature, and it still won't program.
If this helps, I get the following error: avrdude: stk500_getsync(): not in sync: resp=0x15
I am still trying...
Thanks again for the help,
Alan