What does "NFC Connectivity (Optional)" mean for the Samsung I9100 Galaxy S II?
It means some models have the NFC chip, and some don't. From what I understand, at the moment, only the model released in South Korea has the NFC chip. My model (from the UK) doesn't have it.

Though I've been following SGS2 news, I haven't read anything about the possibility of retrofitting a chip later - either you have it or you don't.