Talk about perfect timing. Just when I wanted to take my iPhone 3GS to Sri Lanka, they released the unlock for 3GS. I had iOS5 installed on the 3GS, and the new jailbreak and unlock works for 5.0.1.
So following the guide below, I managed to unlock my 3GS, which is on the new bootrom. BTW, I unlocked it on the MacBook Air, because the PC doesn't have internet yet. :D
Since I don't have a SIM card of another carrier, I thought maybe if it worked fine, it should also recognize the Softbank SIM card without a problem. Maybe it doesn't really mean it is carrier unlocked, but I wanted to check.
So I removed the MicroSIM card from my iPhone 4S, and inserted it to the 3GS using the MicroSIM card adapter I had bought from eBay a while back. When I booted iPhone 3GS, it worked fine. The service was recognized. I was happy. Maybe it didn't mean anything. Only way to find out if it really worked was by using my Mobitel SIM card back in Sri Lanka.