How about a mobile with operator's LBS support when you just launch an app that tells you exactly where you are at the moment.

It sounds simple enough. But already there are some challenges - if the application is not running all the time then when should it be launched? And if it is running all the time e.g. screensaver style application (lets avoid the issues of power consumption and coverage for a moment) how to overlay other important status information such as incoming communications?

And what if the person wants an application showing where they want to get to, rather than where they are?

Simple things become complicated. Knowing more about the user's context and what they are likely to need in any given situation is necessary for many of the potential solutions.

Okay, let's talk about possible implementations.

Afaik, mobile maps have been in use by NTT DoCoMo users for quiete some time. ATT's m-mode also featured maps and mapping friends on maps.

One option I suggested http://www.mobhappy.com/2005/11/social_navigati.html

You have an item in your SIM menu "Where am I?". Upon clicking an sms is then to the carrier with a request to fetch location information. A carrier or LBS provider than makes request about user's current location and sends back an MMS message with a map shot.
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You have figure out where you are, then you plan the route to go from here to your target location.