This questions has come up with a couple of our Salesforce.com clients so we thought we’d share this information.

When using Workflow rules to trigger email alerts, activities are not
automatically logged when the email is sent.  Some users would like a way to
track if and when an email was sent under the activity related list.  To do
this, have the Workflow trigger a task (in addition to the email alert that
is sent). You can assign the task to the record owner, creator, a certain
role, or a specific user. You can also set the task to “Completed” so it
will just be used for tracking purposes.

Once the Workflow is triggered, the email will be sent and a completed
activity will be logged on that particular record simultaneously.