A Swedish legal expert answers:
“The conclusion is: the possibility of extradition exists, but a number of conditions must be met (including that the crime is not political), and it is far from certain that a U.S. extradition request would meet such requirements. In addition, the UK – which hands over Assange to Sweden – has a veto, under Sec 28 of the Framework Decision on the European Arrest Warrant. “
Later on, Associate Professor of Public International Law, Pål Wrange argues:
“On the site http://www.justice4assange.com it is alleged that in the Swedish extradition agreement with the U.S. there is a special summary procedure that so to speak bypasses the usual requirement that the crime may not be political, that the death penalty may not be imposed, etc. On the same site is also stated that there is no such procedure in the US agreement with the UK (see here). Both of these claims are completely wrong. (The other arguments are hardly worthy of comment.) “
Update 1: Mark Klamberg, Doctor in Public International Law, has covered these issues before on his blog, Extradition of Assange to the US via Sweden for espionage.