Another security flaw with the passcode lock has been discovered in the iPhone 2.1 firmware. Thankfully this flaw isn't as major as the last one discovered.
iClarified contributor Mateen has found that if you enable the passcode lock and disable SMS preview, your messages could still be seen!
To duplicate this flaw just go to your settings and enable passcode lock. Make sure to turn off SMS preview. Now sit at the Emergency Dial screen (the only one you can access without the password) and wait for a text message to come in... Surprise! You can read the entire SMS!
Not a major issue but definitely one that should be addressed...