Historically the fish was used as a secret sign by Christians to identify themselves to each other, back when being a Christian meant persecution and possibly execution. The last time I checked the local paper, though, Christians weren't being rounded up and burned alive by the authorities in California. So to me, using the fish symbol smacks of a persecution complex. Twenty or thirty years ago, whenever this trend started, not many people would have known what the fish meant. So it was a way for Christians to nudge-nudge-wink-wink make contact with other Christians without the heathens being any the wiser. Isn't it in Luke where Christ commands his disciples to "go and form secret societies within secular culture and communicate in code so that no one can identify you as one of my followers"?
Go read the rest. Um, unless you have a fish on your car.