The most likely cause of this is not using the correct JDBC driver for your version of MySQL. Eg., if you're using an older version of MySQL, you may need to downgrade the JDBC driver that comes with jForum.
You might upgrade to the latest JDBC driver. A quick google of "c3p0 deadlock mysql 5.1" came up with various MySQL issues about C3P0 and deadlocks. Most seem to be resolved with newer drivers. [originally posted on jforum.net by monroe]