Encryption, OSes, and performance

As a point, both linux and OS X (yes, I know I'm dragging in another contender here) support encrypted swap files. On a Mac, open the System Prefrences utility, go to the Security tab, and click the "Use secure virtual memory" option. Linux users, I'm afraid, don't have it quite so easy - instructions are somewhat distro/kernel specific as of yet, and there aren't any cute little widgets to make life easy for you here.

As far as performance goes, I think most people will find (as I have so far) no noticeable impact at all. If you think about it, a machine that is actively paging is a machine that is most likely IO bound, not CPU starved - so in most cases, much of the encryption will occur in what would otherwise have been wasted clock cycles anyway. This is not necessarily the case in all situations, but will probably be so for the vast majority of users. And if you're paging enough that encrypting your swap file raises real performance concerns, you already have serious performance problems caused by the constant swaping.