Today, I've fully deployed our tracking scheme using Google Latitude. My wife and kids have started their road trip and I'm tracking their progress. Here's how things look so far.
Using the Latitude data, I can determine their position in the past 2 minutes (it appears to update only every 2 minutes). Using two points and their timestamps, I can estimate the approximate speed they were driving between the two points. As I mentioned in a previous post, I'm storing the locations in a MySQL table and through AJAX, I'm able to update a map with their path and show information bubbles over each point with some information, which is what you see in the video linked above. You can see an occasional large gap between points as they move in & out of 3G coverage, but it's not too bad since we don't use AT&T. HA!