He's wanted in Sweden on criminal charges, we have an extradition treaty with Sweden. End of.
The fact that he's hiding in the Ecuadorian embassy under the pretext that those nasty swedes will send him to the States is (IMHO) Boll**cks.
If the Yanks wanted him they would start their own extradition and Cameron would no doubt send the SAS in to the embassy to get himout just to please them.