iphone and contracts (Milan)

How does this work in Italy ? I already own an iphone. Is it possible to arrive in Italy and just buy a contract without having to buy the phone? Sounds like a strange question but here in Japan, you can only buy the iphone from one provider company and you are locked into their contract. The iphone is then 'locked' and can't accept other SIM cards. However, I know that they can be 'unlocked' ( in Hong Kong ) and then other SIM cards can be used. I would really love to get some information. Thanks.