I was able to get this working but I wanted to throw something in for your edification.
This was as a result of using the Linux SNMP wizard where I chose to monitor up/down of MySQL. I didn't specify any warning threshold, but rather only critical (0,2). When I submitted, it all went through fine but with the same MySQL error listed in this thread.
Now that I'm up to 20121.5b I was able to twiddle with the command set and see what worked:
This is what the wizard put in as $ARG1: -C public --v2c -n 'mysqld' -w '' -c '0,2'
This is what works: -C public -n 'mysqld' -c '0,2'