How serious a problem is it to use pressure-treated lumber for borders for raised beds? Is the leaching that extensive? Can p-t be polyurethaned to "seal in" any chemicals? Thanks for any advice.

The only way to seal out the chemical leaching that I know of is to wrap the lumber in heavy gauge plastic sheeting. PTL leaches more in damp conditions. You'd be okay with flowers in those beds, something non-edible.

There is a liquid polymer called EPL http://www.ccaplaygroundsolution.com which can be painted onto the wood to seal it in, also you can use rubberizer paint, if you like, or you can use plastic sheeting inside, and then plant arsenic hyperaccumulators around the base. Do NOT plant anything in the mustard family in a raised bed made with pressure treated wood, as they are arsenic hyperaccumulators and can poison your family.

We are mixing out own treatment for nonpressurized wood. The recipe is from the OG site. Do a search for something like "treated wood" and you should get the article with the recipe. It contains linseed oil, turpintine and parafin wax.