Actually keeping the pass phrase out of swap space is fairly easy (although I havn't looked at the PGP code to see if it actually does this). Read the pass phrase in raw mode, one character at a time and convert it one character at a time to the decryption key for the private RSA key. Then the OS doesn't need to buffer the whole line, either in kernel space or in user space.
This isn't as easy as you think, and it completely breaks many abstractions to do this! PGP does some of this, but not all of it. It does get the pasphrase one character at a time, however it does buffer it all before hashing it to a key. One reason is that you do not necessarily know how big a session key you need, so you may need to use different hashing techniques to get different sized keys. It would be nice if PGP tried to lock its memory pages on OS's that support page locking, so that some pages dont get swapped out. But that isn't a very general solution, since not all OS variants provide such a mechanism. -derek