I've already suggested that you lock your smartphone so that thieves can't steal your business data along with the device. But what if you misplace the phone and someone wants to return it? How thoughtful... They'll be greeted by a prompt for the password, and typically nothing else. Here's how to configure a BlackBerry or iPhone to show your contact info on that screen. Offer a reward for a bigger incentive.
A BlackBerry is better prepared for this situation. You just have to choose Owner in the device options, add your contact info, press the Menu key, and choose Save. The login screen will now include your details.
You'll have to punt on an iPhone, since it doesn't have this handy setting. Create a customized background pattern with your contact info as a work-around. On a PC, open a favorite picture in an image editor, and make the picture 320-pixels wide and 480-pixels tall.
For the most visibility, enter your contact info along a horizontal stripe between 116- and 170-pixels down from the top. This area is clear when the phone is idle and also when entering the passcode. If you want more space, enter your details between 116- and 386-pixels down from the top. Much of this will be obscured by the keypad screen, but the full area is clear when the phone is idle.
Email the photo to yourself on the iPhone. In the email, hold your finger on the picture for a moment until you get the option to Save Image. Choose that, go to the Photos app, and open the image. Tap the button in the lower-left corner, and pick Use As Wallpaper.