IIRC there were some pretty ugly lawsuits over chromium copper arsenate (CCA) impregnated exterior wood a few years ago that had been used in exterior applications like decks and playground fences. Splinters and incidental contact gave a few kids serious poisoning, hence the uproar over the stuff and the changeover to the copper "quat" impregnated woods.
There are still a number of CCA decks, wood foundations, sill plates, and who-knows-what-else out there. Generally speaking they're not a problem unless you get a splinter from the wood or handle the wood as it is leeching material out. You can also seal it (of course if you're going through the time/effort/expense to seal it, it sort of defeats the rationale for exterior grade PT lumber in the first place, but that's another discussion. . .)
Generally speaking the copper quat ACQ stuff is supposed to be reasonably safe although I still wouldn't go making picnic tables or park benches out of it or whatever.
There are also some issues regarding building material recycling with CCA-impregnated lumber. . . A G.C. would probably know more about that than me though.
http://www.sptimes.com/2002/06/15/Homes/Beware_that_picnic_ta.shtml