Do not use pressure-treated lumber for building a raised bed.

Don't use it for building children's playsets. Don't use wood chips from chemically treated lumber as mulch or as cushioning in children's play areas. The pesticides and preservatives in the wood are very strong and can be absorbed through the skin and by ingesting plants that have absorbed the chemicals. Keep it out of the garden and out of direct contact with humans - especially little ones!

Untreated wood will degrade over time but it could be up to ten years before you would need to replace it. At two years old, the untreated redwood boards surrounding our beds have weathered nicely and are quite sound. Make sure to ask when you're shopping for lumber.

Ohio State University Extension Office Factsheet
The author, Robert L. Romig, does not advise against using treated lumber in all cases but does an even-handed job of giving facts about different lumber types you might use, as well as what you should expect from the manufacturer when purchasing.

Is CCA PT lumber OK for garden use? Jim McNelly
This page is from a chat group discussion about the subject in 1994. Mr. McNelly reprints the warnings that you should receive in writing from a manufacturer when purchasing.

From the EPA, some short scientific reports about the impact of the various chemicals in treated lumber on marine life. I'm not a scientist but even I could see words "reduced", "contaminated", and "dying" kept cropping up.
http://www.epa.gov/gbwebdev/ged/publica/c3649.htm
http://www.epa.gov/gbwebdev/ged/publica/c0864.htm
http://www.epa.gov/gbwebdev/ged/publica/c0858.htm

The Nature of Our Garden