what is the best method for device location services
You might be intrested in this RFID tracker. Hack-A-Day has a write up on it here:

http://hackaday.com/2010/02/20/rfid-tracking-system/

The project site is:

http://www.ns-tech.co.uk/blog/2010/02/active-rfid-tracking-system/

Simply put rfid tags on what you want to track, hook up a reciever, and connect it to your computer. It's all open source and on a hobyist budget. I'm sure you could modify the code to use the data for whatever server application you want. I heard of jewelry stores using similar devices to know if merchandise is being stolen. Hope I could point you in the right direction.