Well, it could happen. Back in the day (before PCs), there were a lot of computers that would ship with some features disabled that a technician could "field enable" by moving a few jumper cables. It was just cheaper to manufacture everything the same.
Why would Apple do it? Well, it might reduce some manufacturing costs (but not by much, as Apple needs to pay license fees on several of the 3G related components). Apple does hate SKU bloat, so this would let them trim down the number of different iPad products.
It would be nice to hit the App Store and buy a Sprint, AT&T or Verizon 3G enabling App -- but that may be a few years out.