iPhone: once unlocked, always unlocked? I got an iPhone from a telco carrier two years ago, as at that time, I didn't really know whether I bought the phone unlocked or was it locked to that specific carrier.

All I knew was, I change my telco afterwards, and I could use the new telco's data plan and voice and SMS service without trouble. 

As I want to travel to overseas soon, I am looking forward to buy local 3G SimCards to satisfy my appetite for Internet surfing. I wonder whether I can do so without performing extra steps on my iPhone?

I think when I managed to change from one carrier to another previously, it indicated the phone was unlocked, at least among local carrier, but I am unsure whether this phone will remain unlocked when I travel overseas? Or is there a possibility that although you can change local carrier, but for you to change to an oversea carrier you would need to do some other steps?
SIM Unlocked means SIM unlocked. There is no lock, so you're free to go.