For a few reasons I don't need to discuss, I am strongly considering using [1/2" to 3/4"] W x 6' H cedar or pressure-treated boards for some garden bed edging. 2"-3" of board will be in/under dirt, the other 3"-4" will be above and visible or retaining mulch. It's not clear that if I use pressure-treated boards how concerned I should be with potential leaching of chemicals into the bed. It's also not clear how long I should expect either of these solutions to last before significant rot sets in below ground. Any thoughts or comments?
The chemicals used on pressure treated timber have not been toxic to any significant degree since new regulations came into force in 2003.
However if there is likely to be regular human contact with the timber (e.g. walking on timber decking in bare feet), it is recommended to apply a sealant to avoid direct skin contact with the chemicals.
Whether you want to do that for your garden bed edging is up to you. If you cut the timber after you buy it, you will definitely need to seal the cut surfaces to prevent rot. The easy option is to buy timber that was cut to the size you need before it was pressure treated, of course.
Make sure you get "ground contact grade" pressure treatment. There are two different types of grading system for timber, one for the amount of protection from the pressure treatment, and the other for the cosmetic appearance of the wood (i.e. the presence of knots, holes, cracks, etc). Don't get the two confused!
The life expectancy of pressure treated timber and cedar should be about the same - 10 to 20 years depending on the conditions. Cedar would last longer if you clean and reseal if every 5 years of so, but would you want to dig up all your edging every 5 years to clean and reseal it, just to get a bit more life out of the wood?