If you use Google Apps, you can record the email addresses of the people who fill out your form. To do this, select the checkbox next to 'Automatically collect respondent's yourdomain.com username' when you create the form. Recipients of your survey will see a message at the top of the form explaining that their username will be collected automatically.
You can also require your respondents to sign in to view and fill out a form. This provides an additional layer of security for sensitive forms.
Note: if you're inviting people outside of your domain to fill out your form, make sure you don't require them to sign in; otherwise, they'll get to the domain sign-in page and won't be able to access the form.