A B737-700 did not pressurize after takeoff so the Cabin Altitude Warning horn sounded at 12;000'. After descending to 10;000' the pressurization system began functioning and the flight continued to its destination. Confusion with the Takeoff Configuration Warning horn did occur.

Narrative

Upon passing through about 12;000 feet we received a Cabin Altitude Warning Horn. I donned my mask per the memory item and stopped the climb of the aircraft at 12;000 feet. The Captain proceeded to run through the QRH and talk with ATC to get lower. Once the aircraft was established returning to 10;000 feet; I asked what the problem was. The cabin was steady at 10;000 feet with the packs on high. I asked if the bleeds were off and the Captain stated 'NO'. After a couple of minutes and once the 'off schedule descent' light illuminated; the cabin began re-pressurizing immediately. The flight continued normally afterwards. Other factors included are that this aircraft was under MEL when we got it for an Autopilot detector fault. On the previous flight we had to return to gate for an overhead 'elec' light that required a replacement part. I am not sure what caused the event; but as a side note; when the Cabin Altitude Warning Horn went off I had to pause to think of what the horn meant. I feel that no amount of briefing can replace what we recognize and test as a Takeoff Configuration Warning Horn.