Today, Apple updated the Find My iPhone app. Although the fixes consist of vague “various stability improvement,” it appears that the app does not crash as much went reopening to enter the id and password.
Apple describes the app as follows: “Find My iPhone is free for anyone with an iPhone 4, iPad, or 4th generation iPod touch (iOS 4.2 or later required). Just sign in with your Apple ID on your qualifying device and turn on Find My iPhone to allow you to locate it.”
This is a pretty nifty service. You should download it and try it out.