Lock Timeout on INSERT?

I'm getting a Lock Timeout on an INSERT in AR 1.15.6 -- has anyone
seen lock timeouts on INSERTS?

I have the following models:

I don't, to my knowledge, have any open transactions in the current
thread. I'm running MySQL 5.0.45

Thanks in advance.

MleDistribution has_many :mle_transmission_sessions w/ a fkey on
mle_distribution_id.

ActiveRecord::StatementInvalid: Mysql::Error: Lock wait timeout
exceeded; try restarting transaction: INSERT INTO
transmission_sessions (`archived_recipients_count`, `created_on`,
`session_key`, `special_instructions`, `status`, `completed_on`,
`free_list_id`, `updated_on`, `communication_version`, `priority`,
`type`, `scheduled_for`, `scheduled`, `mle_distribution_id`,
`submitted_on`, `communication_id`, `bcc_list`, `submitted_by`,
`price_quote`) VALUES(NULL, '2008-03-11 14:30:27', '', NULL, 'new',
NULL, NULL, '2008-03-11 14:30:27', 0, 0,
'MleTransmissionSession', NULL, 0, 137, NULL, 104001, NULL, 891, NULL)
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/connection_adapters/abstract_adapter.rb:128:in `log'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/connection_adapters/mysql_adapter.rb:243:in `execute'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/connection_adapters/mysql_adapter.rb:253:in `insert'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/base.rb:1814:in `create_without_callbacks'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/callbacks.rb:254:in `create_without_timestamps'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/timestamp.rb:30:in `create'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/base.rb:1792:in `create_or_update_without_callbacks'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/callbacks.rb:242:in `create_or_update'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/base.rb:1554:in `save_without_validation!'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/validations.rb:762:in `save_without_transactions!'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/transactions.rb:133:in `save!'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/connection_adapters/abstract/database_statements.rb:
59:in `transaction'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/transactions.rb:95:in `transaction'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/transactions.rb:121:in `transaction'
/apps/eisweb/releases/20080306015912/vendor/rails/activerecord/lib/
active_record/transactions.rb:133:in `save!'