Great idea.
In terms of London, where there is a big transport service, it would help leaving home for work to know that you only have a couple of minutes to wait for the bus or train, and similarly when leaving work.

Another thing that could be useful would be a tracker on a bus or overground train (I'm assuming underground trains would block the GPS system). This way you can see which stops you've passed and which ones are still to go, and which is the next one.

This would be useful if you are going somewhere you are not overly familiar with. I had this recently with a job interview where I was using Google Maps to find out where I was and if I should get off the bus. Not as polished obviously.