This is a situation that's been slowly unfolding throughout the Fall 2012. Model S reservation holders looking to buy 40 or 60 kilowatt-hour models will have to wait an extra two months.
Why? Tesla's manufacturing ramp-up wasn't as fast as expected.
Tesla's original manufacturing plans was to produce 5,000 Model S's in 2012, and 20,000 in 2013. http://www.torquenews.com/1075/tesla-announces-delayed-production-dates-some-model-s-variations
Under those original plans the 60 kilowatt-hour model was to already have begun production. Instead that one will go into production in January. The 40 kilowatt-hour model is slipping to March production start.
It means the 2012 production will be in the 3000 range rather than 5000. They still maintain production in 2013 will be 20,000 cars. http://www.torquenews.com/1075/teslas-q3-had-steep-losses-predicts-rapid-revenue-gains-q4-and-beyond