The Android Market was featuring a great little app called Shush! recently, and I love it. All it does is detect when you've put the phone in silent or vibrate mode, and asks you when you'd like that to get turned off. You can easily hit a button to tell it to leave it on. It adds one more tap to silence your phone, but it's nice to not have to remember to turn the ringer back on, which would require at least another screen tap anyway.