That's what's happened here: Sweden wants to question Assange, with a view to potentially bringing charges. They have issued an arrest warrant to arrest him - his guilt is irrelevant.
As many people have mentioned, the UK actually has a strong record of extraditing people to the US: if anything, Sweden is less co-operative (for example, Roman Polanski will never set foot in the UK for fear of being arrested and extradited, but he was happy to enter Sweden).