How to Safely Put an Alphanumeric Passcode on your iPhone
Uneasy silence published a great write-up on how to put a stronger unlock passcode on your iPhone than the standard four-digit code that is used by default:
First Download the iPhone Configuration Utility from Apple (for Mac or PC, and connect your phone.
Then launch the utility, go to “Configuration Profiles”, “New”. Give it a name and identifier, then go down to Passcode, enable it and set your own options.
Once you’re done, go to your phone in the left pane, “Configuration Profiles” tab, and click install next your profile. It’ll pop up on your phone and you can install it!
Simple and secure way to lock down your iPhone a little more.
Update: iClarified has a more detailed write-up on the procedure.