Hey Ecoxotic,
I've tested this issue by creating a simple web form with the LeadSource field. In the Edit Field Description prompt, set the Default Value to 'Web', click the Hidden checkbox, and update the field by clicking Update. Lastly, click finish to save those changes. I've attached a screenshot of the Edit Field Desciption prompt LeadSource field.
*Note: For Web Form changes to take effect you must copy the new HTML into your desired webpage.
Sam