I think whether a cross is an idol depends on the intention of the wearer, frankly. IF the cross is being worn as a marker, so that other Christians can identify the wearer and non-Christians will be encouraged to ask about the faith ("I will make you fishers of men"), then it's not an idol, IMO. IF the cross is being worn as a deliberate portion of the wearer's religious practice -- they MUST wear the cross -- then it is.

And, regardless of what one thinks about Jesus being a portion of God (and therefore exempt from the NIV interpretation of "it's only bad if it's not really God), the cross is a natural object that isn't God. So the part that fails the First Commandment isn't Jesus, it's the cross that's holding him up. :O

I think all of this forms a part of why I'm no longer Christian.