Is it normal for pressure-treated lumber to warp?
Treated wood doesn't always twist/warp/shrink, but in my experience more often than not it will to some degree. For something small like window planter boxes etc. I would use non-treated lumber and stain or paint with a quality exterior grade product. Obviously if the project is a deck or similar structure subjected to the inevitable wrath of Mother Nature then you should always use PT. Or as Kris suggested you could elevate and stack PT lumber to dry under controlled circumstances to mitigate warping.