BackupPC has been installed on QNAP devices (
http://wiki.qnap.com/wiki/How_to_install_the_BackupPC_application), but there's a very real chance that bi-monthly firmware upgrades will break it. That and the fact that we're waiting for QNAP to upgrade some packages to close some security holes means that it wasn't worth it for me to sit on a particular firmware for a long period of time.
My BackupPC server (dedicated old hardware) writes the pool to a NFS-mounted share on the NAS. All of the things that Arnold mentioned apply ... in all honesty, I sometimes feel that it would be better to just take an old desktop box, dump a bunch of 3TB+ drives in, set up a software raid and let it do it's thing.
It'd certainly be cheaper. Heck, for the price of some of these NAS's, you can build your 2nd server to backup your primary BackupPC server.