As far as I know there are no programs for Linux that provide encryption of the whole operating system. There are individual utilities like PGP that may provide encryption for indiviual files but none that I know of that do so for the whole OS.
Whereas for Windows there are quite a few such programs available (e.g. check out http://www.securstar.com/products_drivecryptpp.php )
Only when such programs become available for Linux will I consider switching. Actually it's a bit strange that there don't seem to be any such progs given the privacy oriented mindset of most open sourcers. Does anyone know if there any projects on the horizon which are trying to address this issue?