Apple has produced a well done iOS app for conventiently location an iPhone, sending it a message, locking it or wiping it completely. TidBITS has the best overview ...
TidBITS iPhone iPad iPod: Find My iPhone App Enables Mobile Device DiscoveryIt's hard to imagine now, but the BB Pearl of 2008 was a proto-gPhone. It ran Google Mobile and you could track it on Google Maps. I came to despise the phone and I was delighted to get rid of it, but I missed the free location tracking. Now I have an easy way to track a phone; unlike Latitude it's strictly on demand tracking - the phone isn't routinely updating its location.
... To enable Find My iPhone/iPad/iPod touch on a given device, tap Settings > Mail > Contacts, Settings, and tap your MobileMe account under Accounts. Set the Find My iPhone switch to On, if it isn't already, and tap Done. In the Fetch New Data view, either or both Push and Fetch need to be active. If Push is set to Off and Fetch is set to Manually, Find My iPhone cannot work, because the location is never updated. (Apple recommends Push for MobileMe.)...
I tested the new iPhone app on Emily and my phones -- and discovered I hadn't enabled tracking on my phone. That was an oversight! I was lucky to discover my mistake because you can't enable tracking remotely, though you can wipe a phone remotely. (For reasons good or greedy, Apple has a far more conservative attitude towards location sharing than Google.)
The new app with MobileMe services is a good way to find a lost phone, or to see how far your spouse is from airport pickup (though you need his/her MM password -- again, conservative on the privacy). However it only works if you enable it, and you need a MM account.
A friend of mine came up with another approach that doesn't require a MM account -- or any advance planning. When he lost his iPhone bicycling to work he had AT&T add the family tracking feature to his account. You can add it at any time, and you don't need possession of the phone (it does have to be on however). This is less precise than MobileMe tracking since it relies on cell tower triangulation, but in urban areas that can work well. It worked very well for him -- he found his phone quickly. He then canceled the service; AT&T allows customers to add and remove services as needed. The AT&T locator service, of course, works with any phone.
So there are two approaches, but if you're primarily concerned about finding a lost phone the on-demand AT&T tracking service is quite a bit cheaper than MobileMe -- and it doesn't require setup. If you have a MM account for other reasons (it's really a pretty crummy service to be honest) this is well worth enabling and testing.