Can anyone think of a way that RSA-the-company could include some sort of RSA-cryptosystem-secured check in a release of PGP so that it would only interoperate with other versions that have the same check? I don't see how it could be done, but it's sure an intriguing idea.

PGP is released in source code, therefore anything that gets put into the code could always be taken out or matched in another version. For example, PGP 2.6 contains the hack to change the version number of packets on September 1. This was necessary to please RSA, the company. And look what happened, 2.6ui was created which matches the functionality (in that it can read the packets that 2.6 will generate after 1-September). The point of this is, why would *you* care? I can understand why RSA _might_ care, but I don't see Phil Zimmermann agreeing to it, and I don't see how anyone could force it into PGP at this point. -derek