lion brand says: Acrylic yarn, or yarn that has a high acrylic content, does not usually block well. The best way to block acrylic is to throw your project in the washer and dryer, following label directions, which will help even out tension." (http://www.lionbrand.com/cgi-bin/lionbrand/displayCustomerProjects.fcgi?projectKey=91816&displayType=lit)
but others, including the following link, say "Acrylic CAN be blocked! In fact, it should be blocked. From my observations, I would say someone along the line made an acrylic sweater, blanket, or whatnot, blocked it in their usual 'wet blocking' manner and it didn't work. Therefore, acrylic doesn't block-warn everybody. No, it can be blocked. You're just doing it wrong." (hint, she advocates for steam blocking - http://beadknitter.blogspot.com/2009/03/you-can-block-acrylic.html)
hope that helps...