IPhone – How to use the US-iPhone in South Korea

Yes. No it won't affect your phone.

An iPhone requires a SIM to activate. If the phone was locked to a specific carrier, it would require a SIM from that respective service provider. Since your iPhone is factory unlocked, any SIM (provided it's a micro-SIM) will suffice for activation. SIM cards to do not affect your device. You could get SIM cards from Fido, Rogers, Bell, AT&T, and Verizon and use each one every hour if you wish. This is why "world phones" (devices that are not locked to a specific carrier) are appealing to those that travel. They can use one SIM card in North America, for example, and another one in Asia during their travels.

SIMs only allow your device to connect to the carrier's towers, that is all. That's what SIMs are for, to talk to the carrier and say, "Hey, I'm authorized to be here." Older technology (CDMA) required direct device activation. This meant having to call the carrier and get the device activated. Should you change your device, you would have to call again to activate the new one. SIM cards (for the newer GSM network) solved this problem by registering the SIM and not the device. Once a SIM is registered, any device its plugged into has access to that network.

If the phone is unlocked, you can use it with any SIM, even from a different GSM operator or even a SIM that is not in service (obviously in this case it won't work as a phone). If you don't have one or can't find one, you may be able to get prepaid SIM for cheap from an GSM operator.