I'm a little shocked myself but I doubt that a replaceable ssd will be on the market anytime soon. Even when it does happen, will the read/write speeds be equal or greater than what Apple's already putting in? It's already cutting edge. Also, will it actually be cheaper than just upgrading it when ordering? Somehow, I get the feeling that the savings will be minimal.
It's not something to I feel most people will appreciate at the moment, but perhaps down the road, if someone who orders a 256 or 512 that needs more space, obviously upgrading the SSD will be much cheaper than buying a whole new machine, but I don't feel as though people should buy these, hoping that third party SSDs will be comparable in quality and cheaper.