"Dollar Cost Averaging" is one the most misused terms out there. An example illustrating a true dollar-cost-averaging decision:
I have $12K in cash right now. Do I invest $12K today, or do I invest $1K per month for 12 months? As everyone on this thread has pointed out, you're likely to do better by investing $12K today.
However, when you are sitting talking to a financial advisor, "Dollar Cost Averaging" is a marketing technique. You don't have $12K today, or maybe you do but you are reluctant to invest. The advisor's job is to get you to invest with him so he can earn a commission. You probably could commit to $1K / month for 12 months, so the advisor starts telling you about how "if the market goes down, you'll buy more shares and have a lower cost basis." Not strictly a lie, but the goal here is not to get you to invest in the best way to make money for you. It is to get you to invest something with that advisor so that he gets his money.
So many people have been sold this way that the statement "DCA = regular periodic investing" is just taken as fact. And even extended to DCA is better than lump sum, even when I really do have a choice.
/end rant