Firstly let me tell you that Itunes never updates your iphones firmware automatically and you actually have to agree to terms and agreements and click next a few times so its pretty hard to do by accident unless you see the firmware and think its ok to upgrade so the mistake being that you thought the firmware would be ok but to actually update by accident is not so easy..
any way if you let Itunes upgrade officially then it will have upgraded you to firmware 4.3.5 with what ever baseband that comes with, but there is no unlock tool for this current firmware and you made the mistake of upgrading firmware so you can either see if you can use an official sim card with the iphone so it will work or maybe even get in touch with the carrier and see about an unlock or look into a sim solution such as gevey sim...
OR you can wait and hope the devs come up with an unlock solution for your baseband any time soon.