To create a contact form that will allow your users to sign up to a mailing list when they want, but to be able to use the subscription form as a contact form whether they want to sign up or not simply follow these simple steps:
Create two mailing lists:
1. General Inquiries
Create a new Email Confirmation Set (on the Lists menu), assign it to your "General Inquiries" list and configure it to not confirm an Opt-In. (Why: You do not want people receiving a confirmation email for the contact form.)
Create a sign up form on the Integration > Subscription Forms page. The new form should be a "Subscribe Only" form, select the 2 lists you created, and select the "eneral Inquiries checkbox field to a hidden input field in the HTML.
Now, when a user fills in the contact form, they'll be given the option to sign up to the newsletter or not. The General Inquiries will always collect their information, so you can do whatever you wanted with it (try maybe an autoresponder to follow up with them after a month or so? eg. "Hi %FIRSTNAME%, Just wondering if there was anything else I could help you with?") and the newsletter option will have people who want to be on your newsletter.