Android Device Manager, the service that helps you locate and lock lost devices, is picking up an update on the web and for Android that gives you better options when locking a device. Now from both the web and the updated Android app, when you go to forcibly lock a phone you own, you can add both a message and a phone number for the person with the phone to call you with.
The callback number is obscured when it's put on the lockscreen, and the phone simply offers a "call owner" button to hit. The message area shows up above, and can display any text you want to put in it.
The message and callback features are optional, but we'd highly recommend using them — if a person with your phone can simply give you a call at another number and then link up to get the device back, it's drastically more useful. You can try it out from the newest Android Device Manager app or on the web at android.com/devicemanager