A phone number has appeared online that, when called from an iPhone, reveals information about the phone's signal strength, the cell tower it's connected to, the codec it's using to compress your voice (depending on available bandwidth), a unique identifier that appears to identify the touchscreen, and more.
To access this information from an iPhone, you would enter *3001#12345#* and then touch the Call button, putting the phone into "field test mode."
However, you would do so at your own risk; it's unclear whether entering field test mode could be potentially damaging under certain circumstances.
More screenshots of the iPhone field test mode are available on wirelessinfo.
*(**via mobilewhack; *code from macoshints; image from wirelessinfo)
