If you are using email encryption on the Mac you are very likely using S/MIME, e.g. with a Verisign certificate. Unfortunately your certificate is not automatically synchronized with your iDevice so that you can read your encrypted messages also on-the-go. But, it’s finally possible with iOS 5. Ars Technica has a good article about how to secure your email communication and make it work also with your iPhone or iPad. Read it here.