This is no different than on the iPad. And as with the iPad, finding a security hole to work around the signature requirements is very simple compared to actually porting Linux. Indeed, as the Surface is a new product, I'd guess it might actually be much easier to find such a flaw than it has been on the iPad — which has had multiple such flaws discovered and exploited (http://en.wikipedia.org/wiki/IOS_jailbreaking#Recent_release...).