Tele Atlas announced a new dataset with historic traffic data a few weeks ago to little fanfare. The reason? No one was really using it yet (save TomTom, TA's owner, internally).
Now TomTom is using the database in a beta version of an online routing program. You can sign up to be in the program here. (There's also a rather long video introducing the site.) The big differentiator is that the algorithms will tap into current and past data about traffic (say, how busy it typically is on a Sunday afternoon between 6-7 pm) to produce a better route.
It will be a big score if users note a significant difference in the accuracy of routes over those not using this database. We'll see how it plays out.