How can I troubleshoot MySQL Lock Timeout Errors with Rails?

All of a sudden (without any changes to related code) we are getting lock errors through active record such as:

ActiveRecord::StatementInvalid: Mysql2::Error: Lock wait timeout exceeded; 
try restarting transaction: UPDATE `items` SET `state` = 'reserved', `updated_at` = '2012-09-15 17:58:21' WHERE `items`.`id` = 248220

and

ActiveRecord::StatementInvalid: Mysql2::Error: Lock wait timeout exceeded; 
try restarting transaction: DELETE FROM `sessions` WHERE `sessions`.`id` = 41997883

We aren't doing our own transactions in either of these models, so the only transactions are the built in rails ones. There has not been a surge in traffic or request volume.

These errors appear to be when a "new" query tries to run on a locked table and has to wait, how do we see what it's waiting for? How do we figure out which part of our code is issuing queries that lock the tables for extended periods of time?

Any ideas on where we can look or how to investigate the cause of this?