Does the cal event already exist or are you creating it off the email?
If your creating the even off the email then if you mouse over the date/time in the email you will be able to create an cal event for it. The event will contain a link back to the email.
You can also drag a message from mail and drop it on a day and an event will be created with a link to the mail.
If you have an event already created open the Info window on the event and drag the email to the URL field and a link to the email will be added.
Message was edited by: Frank Caggiano