FWIW, one thing that is emphasized in most of the churches that I've attended is standing up and not hiding your faith. In fact, if we truly are saved from our sins it makes perfect sense that we’d want to proclaim our identity in Christ for the entire world to see. For many this means wearing T-Shirts, putting bumper stickers and fish on cars, and yes, fish on business cards and other such things. It is a way to show others that we are not Christians only on Sunday mornings when we go to church, but in everything we do. Of course it goes without saying that for this to be true our faith must be lived beyond wearing a symbol of identify- that kind of hypocrisy speaks volumes and is a huge turn off to non Christians (as evidenced in this thread). I realize that there have been those who wish to be treated differently or favored because they are Christians, but the vast majority of people I know who do all of the above are sincere in trying to communicate their identity in all that they do, not win business.

Personally, I don't generally don't rely on external labels to identify myself, except for sometimes wearing a cross around my neck (which doesn't always identify a Christian anyway- this is more of a personal thing to focus my thoughts during the day). There are multiple reasons I don't use symbols, certainly near the top is the fact that they are ambiguous and often don't leave much room for personal interaction (not to mention often they are tacky or even wrong in the case of T-Shirts and bumper stickers). Another drawback is that people assume they know all they need to about you when they see that fish (once again, this thread is an evidence of that). I generally haven't had difficulty demonstrating to others that I'm a Christian without having any symbolism around me- even without explicitly talking about God around people they soon figure it out. None of this is to say I don't think Christians should have symbols; they just aren't for me. For some it is a natural and valid way to express themselves.