I had a Tom Tom about 5 years ago that knew the speed limit on major roads. I'd imagine Google et. al. are collecting that data even if it's not publicly available in their mapping APIs yet.
They are. All phones who have Google Maps installed, and have enabled reporting (anonymous) position data (might be default on some phones) are helping Google both see if there's a traffic jam somewhere, and see what the actual driving speeds on the roads are (by sampling at a certain interval and seeing distance traveled over time).
I happily contribute to this, I think it's a great idea to crowd-source traffic data. I do hope Google makes the data publicly available though.