… in my inbox today:
Thank you for being a long-time contributor to the OpenStreetMap project. We are writing to ask you to login into your account at http://openstreetmap.org/user/terms and accept a new set of contributor terms.
Why are we asking this?
We need to ask you to explicitly grant permission for us to continue using your existing contributions under the upgraded license and terms.
The new OpenStreetMap data license and contributor terms is a big improvement for the OpenStreetMap project and will encourage greater use of our data. The new Attribution and Share Alike license, ODbL, is written specifically for databases and applies better and more evenly to data in all legal jurisdictions around the world.
Over 121,000 contributors have already accepted the new terms and we hope you will too. Visit http://openstreetmap.org/user/terms to read and accept the new terms. You can read more about the change on the OpenStreetMap foundation site in English and several other languages.
The issue with old license is primarily that it has been written for protecting creative works, but not data. When a somebody uses OSM data in the production of a map or map book, it is unclear to what the OSM CC-BY license extends to: the layer in the map, the map as a whole, the map book?
In any case, OSM has an informative wiki page on the change and one on the new Open Database License (ODbL). More information on ODbL can be found at Open Data Commons. There is also an overview of OpenStreetMap ODbL acceptance by region and the full new OSM contributor terms are detailled here, a summarised version can be found here.
And the process of accepting the new license is hassle-free.