Yes, it's true. You can identify the models that have this restriction by a blue dot on the box. See this article from engadget.
http://www.engadget.com/2011/05/20/p...or-music-othe/
Apparently B&N has also partitioned the new Nook Touch reader to have similar limitations on sideloading, which basically forces you to use an SD card for your personal content.