Head to Settings> Applications> Maps> Check for updates and you will find a update waiting to be downloaded on your Windows Phone.
The new data appears to be available in many locations including USA and Europe. As Nokia uses a system which only sends updates rather than the whole maps it is likely the download will be small, so it should not cause too much of an inconvenience making sure that road is actually not one-way.
Here Maps data are owned by Nokia and are widely used in in-car navigation systems and also by numerous 3rd party GPS Navigation apps like Co-Pilot.
We are installing the update. Do let us know if you find anything new in the latest update.