Also of note: http://en.wikipedia.org/wiki/Miranda_rights#Application_of_t...
It's only necessary to mirandize someone prior to interrogation, it does not have to happen immediately at the point of apprehension.
Additionally, there are some reports that Tsarnaev was, in fact, mirandized when he was taken into custody.
As an aside, I find it ridiculous that anyone is wanting to treat this suspect as an "enemy combatant". What possible advantage is there to that? What legal justification? My take is that people want to short circuit the justice system and I find it appalling that such a tendency is even remotely commonplace.