Got a lock wait timeout originating in Mage_Sales_Model_Order::_afterSave():
https://github.com/LokeyCoding/magento-mirror/blob/magento-1.8/app/code/core/Mage/Sales/Model/Order.php#L2152
Looks like it's due to a reporting query that was running for a very long time and keeping the lock for a long time:
INSERT INTO `sales_order_aggregated_updated` (`period`, `store_id`, `order_status`, `orders_count`, `total_qty_ordered`, `total_qty_invoiced`, `total_income_amount`, `total_revenue_amount`, `total_profit_amount`, `total_invoiced_amount`, `total_canceled_amount`, `total_paid_amount`, `total_refunded_amount`, `total_tax_amount`, `total_tax_amount_actual`, `total_shipping_amount`, `total_shipping_amount_actual`, `total_discount_amount`, `total_discount_amount_actual`, `exchanges`, `gifts`)
...
Haven't dug into it much yet, was thinking I should post up a question before diving in. My inclination is to just disable the report aggregation crons entirely and rely on simple order queries (a la Aheadworks Advanced Reports) for reporting.