The chemicals used to treat the wood will leach into your soil and can end up in plant tissue (and there are scientific studies to confirm this). PT lumber used to be treated with arsenic, but this was phased out in 2003. Now it is usually treated with chromated copper arsenate (CCA) which the EPA classifies as a restricted use pesticide. Whether or not the PT lumber will leach enough toxins into the soil to affect plant and human health depends on many factors so it can be hard to give an exact answer to how safe it is to use in a garden.
Personally, I think it has no place in a garden, especially and organic garden. Do you really want to be smoking chromated copper arsenate? Again, the amounts of CCA needed to have a negative effect on human health depends on many factors, but do you really want to take the risk? If you use it, CCA *will* be in your soil and it *will* be in any plant whose roots come within a few inches of the wood.
Redwood coated with tree seal on the inside is what I use for my raised beds, it will last for a good 10 years. Redwood does tend to be expensive though, especially through Home Depot.
