WikiSPEEDia, the open speed limit database.

Google navigation app (on Android) does not have speed limit info.
Some other app is needed for that...

Here is one (potential) solution:
Welcome to the Wikispeedia, the open speed limit database.

We are gathering the GPS-coordinates of every speed-limit sign on earth.
You don't need any GPS or cellphone to help, just 2 minutes of your time.
Its open source so you can contribute.
People can put these data into GPS devices to regulate or warn of excessive speed. 
Google, Garmin, Teleatlas, even AAA aren't doing this, so please help out. 
Plus the data you capture is copyrighted with your name.

It even has open source code
Nice idea, if it works, and can you trust such data?

As usual, there is a simpler solution for some money...
TeleNav "Premium" has such feature, and more... Having Google as competition is not fun...
So, TeleNav is now available as free app, with limited features...