This idea came from following atamariya's work on CUE:
http://talk.maemo.org/showthread.php?t=81331

He is developing an application to emit an alarm at a location based on cellid based positioning to consume less power. It has a configurable radius for the alarm via the user interface. I was thinking that this was clever, because that location data should even if coarse be good enough via the radius setting to make it much more flexible: as it matured, the action could be selectable as an alarm, or a profile change, or even execution of a command-line function. For example, a profile could be set to silent when at a library, hospital, lecture hall, etc., & back to general at the parking lot.

What would be excluded from those location-based possibilities are things like my forgetting to turn on FM Carkit when I get in the car.

I was thinking that I have an inexpensive ebay OBD2 Bluetooth dongle attached to the car & listed as a trusted device in the N900, & if the N900 detected when it was in range of that particular trusted device, if it would be able to execute a command-line function - like turn on FM Carkit, bring up a mapping application, switch to another profile, etc.

The N900's bluetooth would have to be left on, that would consume a little more power, but I typically charge the phone in the car as well as at home, so it might not be an issue.

It could also be used to change profiles, execute commands, etc. when near trusted BT devices located in other areas like at work, at home - thus mimicking RFID tag detection. BT range is shorter than the grosser positioning of location via cellid, so the two would complement each other well for location-based actions.