the stock linux kernel has a driver which exactly such a thing, called network block device. people are using it, plus software raid, in production environments. i've heard of it used more often for raid-0 striping (eg to make petabyte filesystems out of a cluster of PC's for free) than raid-5, but it is capable for the full range of raid types that linux's software raid driver supports. i tested it once with raid-1 just for giggles and the performance was acceptable for home directory service (mp3 streaming was probably my highest bandwidth activity), but only have one "always-on" server so it wasn't practical long term.