Magento guest checkouts are great for allowing customers to quickly pay for goods without registering but not so good if the customer decides at a later date to create an account at your store and order some more products. When they login to their newly created account they don't see the previous orders they made using the guest checkout.
This is not only frustrating for the customer but also for the merchant as it makes it difficult to assess customer spend and carry out basic CRM activities such as providing support for previously purchased products. To solve this, I have created a very simple module that performs a search for orphaned / guest checkout orders related to a customer's email address. The search is activated when the customer signs into your store and if orphaned orders are found that match the customer's email address they are assigned to the customer's account, so both the customer and store administrator can view purchase histories and transactions.
Download the module for free and let me know if you have questions or comments!