How do you use a users email in a form?

Ok so I have successfully modified the MarketPress Shipping form to only show specific Cities from a dropdown <select> where it normally has a text field instead. This allows me to restrict my service to a specific region.

The email field by default is

<input size="35" name="email" type="text" value="" />

What should I use to fill in the value with the email of the user account from wordpress? I do not want this field to be editable also. It's meant to ensure I can track a purchase to a specific account in wordpress.

I'll be looking out for this solution in the meantime but any advice would be great :smiley: