I’ve found a massive flaw with the iPhone. It doesn’t sync email certificates held in the Mac OS X keychain to the iPhone.

Whilst this isn’t initially a problem if you just use it to sign emails as being from you, it poses a problem if someone else also uses an email certificate enabling you to encrypt emails.
The email ends up encrypted and the iPhone has no way to decrypt it because it doesn’t possess any of the certificates that Mail does. I’ve filed an enhancement request with Apple, let’s hope someone picks up on it.