I just want to post my experience here I think quite a lot of people might have faced the same problem. I recently got an Iphone 3GS and tried to jailbreak using Blackra1n. This was successful but I couldnt sync anything through itunes. I tried jailbreaking using Redsn0w and I got stuck in DFU mode. I had no choice but to update to 3.1.3. So warning for those jailbreaking a recent iphone 3GS hold back for a while or do lots of investigation before attempting to jail break it.