I was experimenting with options for iPhone passwords - those enforced from the Exchange server.  I created a custom mailbox policy that required alphanumeric passwords.  I fiddled with it a while to see what the options meant and then went back to the original policy that just required a 4-digit PIN.  However, I was unable to go back to a numeric PIN (it kept requiring 4 characters including a special character) until I Reset All the iPhone settings (which erased my e-mail setup, network settings - including the WPA key, etc.).

So, if you're tempted to test a longer and/or alphanumeric and/or complex password on your iPhone and may want to go back to what you originally had, be prepared to Reset the phone (you don't lose applications or data but you lose all your custom settings).