File this under "it should have been free all along." Apple's "Find my iPhone" feature, previously a feature of the subscription-based MobileMe service, is now free--with a catch: it's only free for users of the iPhone 4, iPad, or newest fourth-generation iPod Touch.
The service finds your iPhone, if by "find" you mean "get a rough GPS estimate of the location." Still, the service has had its success stories.
Its two best features are, undoubtedly, its ability to remote-wipe all data if necessary, and its triggering of a played-back sound even if the device in question is set to silent. The latter has actually saved my apartment sanity on several occasions.
Even though "Find my iPhone" requires certain settings to be activated on the iDevice in question, and can be relatively easily thwarted by thieves, consider this: you need not fear losing your iPhone 4 under a sofa ever again.
iOS 4.2 will be live later today.