Hi Robb,
Short answer: yes
Marketo must have an associated cookie (on the prospect's browser) associated with their lead record in Marketo. That can happen via two methods.
1) Click tracking link in email
2) Fill out Marketo form
Clicking a tracking link in email is why my above comment is important. You promted me to create a separate thread yesterday here: https://community.marketo.com/MarketoDiscussionDetail?id=90650000000Pm26AAC
Basically, as long as a link is converted to "tracking code" then Marketo associates the cookie on the browser with that lead record. You can tell someone is cookied by looking at the activity log for their record. You will see "clicked link in email" and then "visits webpage" activities (on pages that have the munchkin tracking code).
When someone is cookied, then Marketo will prepopulate available data into forms automatically (though you can turn it off manually if needed).
Hope that helps.