The guys at 9to5Mac have put together a profile you can install on your iPhone using a corporate development kit that gives you an alphanumeric passcode lock—versus the standard 4-digit PIN-style lock.
The settings profile's easily removed if you don't like it, but make sure you don't lock yourself out of your phone by forgetting the passcode. Also, it's entirely at your own risk, you know, in case it does blow up your phone forever.