There is a shipment tracking thread for SSD iMacs, and so far it doesn't look like any BTO iMacs with SSD have shipped at all, and there doesn't appear to be any solid information on the reason for the delay.
But if you want it, why not just buy it and wait it out? It's not going to get to you any faster if you wait to order.
Intel released some new SSD on wednesday, whether or not these will be in the new Imacs remains to be seen. (hopefully) This could be the cause of the delay. If Apple was using the same SSD as the 2010 Imac why would there be a shipping delay.