My origninal thoughts about locating cattle, etc. were to use the method we use for locating underwater devices: triangulation of three angles derived from the phase relationships of signals sent from the tracked object to three (or more) base stations. This method is in contrast to JBD's method of calculating position by the tracked object of time differences of signals sent from the base stations. The time difference (again using phase relationships) give distances. Three distances (like three angles) from base stations with known positions will give a location. The resulting location is sent along with data and identification back to the base network.
The problems we have seen with both of these, distance or angle, methods is in the size, power, and cost of the "tags."
What I currently think might be a better technology to study is the use of Passive RFID (Radio Frequency ID) technology, the method used by the EZ-Pass Highway toll collection, and numerous biology data collection projects (see http://www.fs.fed.us/eng/pubs/html/98241201/98241201.htm and http://en.wikipedia.org/wiki/Transponder_timing). The greatest benefit is the size, power, and cost of the tags is reduced to such an extent that it becomes feasible to scale the number of track objects up to the thousands. The down side is that the distance from scanner to tag is quite small and therefore there need to be more, but it is not unreasonable (see http://www.retailtechnologyreview.com/absolutenm/templates/retail_rfid.aspx?articleid=642&zoneid=2 and http://members.surfbest.net/eaglesnest/rfidspct.htm). If, instead of using UHF, using VHF frequencies, the range can be as much as 100 meters.
It is conceivable, though I haven't researched this, that by using the mesh network of base stations with known locations and the relative transponder signal strength and/or two-way transponder response time, along with a bunch of statistics, that fairly precise location can be determined. Isn't this how our cell phones can give location without using GPS?