MySociety.org has an article, with examples, about travel-time maps and their uses. The page is the result of work funded by the UK Department of Transport and shows a number of final maps created for showing travel times.
Of the more interesting tidbits is the comment that some of the maps consumed up to 10 hours of computer time to create, which makes them something you won't be doing as part of a Google mash-up anytime soon.
However, the maps shown are very interesting and potentially useful for travelers who are trying to decide whether to use rail or road (or other means) to take a trip. In particular, the comparison map showing the differences between rail and road travel times is interesting due to the patterns created, although I imagine not particularly shocking as the train travel is most often faster in areas nearest major cities and slower elsewhere.