It could well be that Apple is announcing something new and that they ran out of the old configurations, so they can't ship the updated models. It could be just something relatively simple as iLife 2010, but then that might affect the 21.5" models as well and they are shipping normally.
I ordered on January 4th with expected shipping on the 18th. I called Apple Store Nordic on the 18th and received an email saying that the shipment would be delayed by 3-4 business days. The same night, the online order page bumped my shipment forward by two weeks, which is where it is now. The US and Finnish stores give the same dates, but with slightly different wording:
US:
Prepared for Shipment
Ships: 01 Feb, 2010
Delivers: 05 Feb, 2010
FI:
Not yet shipped
Estimated Shipping: 01 Feb, 2010
Estimated Delivery: 05 Feb, 2010
If the machine is indeed "prepared for shipment", then it is either being held back because it needs to be announced first or because there are shipping delays. Two week shipping delays would seem quite extraordinary though, especially considering that the 21.5" models are shipping normally.