Fair enough. For now it's up to the receiver to decide wether they're going to validate a signature. That goes for S/MIME and PGP both. S/MIME can be easily used on multiple devices, though, since they are simple X.509 certificates. I have the same certificate installed on both my computer and my iPhone. For added security you can even put it on an industry standard smartcard and your OS will pick it up from there automatically (after entering a PIN probably). It's all in there.