It's been done, and here it is:
Courtesy Uchida, H. and Nelson, A. Agglomeration Index: Towards a New Measure of Urban Concentration, who came up with this for the World Bank’s World Development Report 2009. Large version here.
Now the major caveat that this map is done with a very simplified model: a city of at least 50,000 people is bright yellow, and as the distance from all cities increases, the color changes to red (24 hours from a major city) and then black (10 days). The white lines are major cargo shipping routes, which is useful if you're a container, but less so if you're a human.
So this doesn't actually account for roads, flight routes, etc, it just approximates them by assuming that lots of people = lots of transport options. Fortunately this is mostly true, although it's not hard to find bugs: eg. coastal Greenland and most of Papua New Guinea is actually fairly easily accessed by plane, whereas much of the Sahara is not, and traveling in India isn't as easy or quick as traveling in (say) central Europe.
(Edit: Oops, just realized SpaceDog already posted this in a comment several days ago. But IMHO it's worth a full answer!)
For travel time maps that uses actual connectivity information, but only work in some cities, check out TriptropNYC (pretty, but very slow and NYC only), Transit Time NYC (rougher approximations, fast, again NYC only) and Mapnificent (lots of cities, but only shows a 'boundary' of how far you can get in X minutes).