On the longer term, cookies are an endangered species. Goverments tend to ban them and users to, more and more often erase them.
What about considering alternatives to cookies to recognize a recurring visitor.
The Electronic Frontier Foundation has released a study a while ago that releals that popular browsers give Web sites enough information to get a unique picture of their visitors about 94 percent of the time. Which is better than cookies today.
They even have released a tool to demonstrate their point. : http://panopticlick.eff.org/
It would be interesting on the longer term that Marketo consider this alternative to cookies.