I would think that lowered December sales makes sense given the timing and manufacturing lead times. December sales are too late for Christmas and too early for spring/summer consumer spending. If I was guessing I would think that Feb/March would be much better months for sales if the consumer markets hold on or are expected to grow for 2013. This
You are right that foundry sales normally drop sequentially in December.
However. in TSMC's case over the last ten years that has typically been by single-digit percentage rather than a double-digit percentage.
In 2011 TSMC invested heavily in switching technology nodes from 40nm to 28nm.
This figure of 16% drop of profit is an improvement of year before of 22% drop, shows they are already recovering.
Now waiting for EUV or Ebeam for 14nm node for next big slump.