The key point, an FAQ at Singtel's website:
"Will my iPhone work with a StarHub or M1 SIM?
It may work but you may not enjoy the same iPhone experience as SingTel only supports iPhone 3G devices that are connected to SingTel’s network"
So, iPhone 3G in Singapore is probably unlocked out of the box. More proof that cellular business in the US is ancient and backward with all the locking crap.