as probably all of you know by now (as it has been discuses in many places including this forum), there are plans to change the OpenStreetMap license from the current CC-BY-SA to the OdBL. As has also been discussed at length, this change is mostly necessitated by the fact that the current license is not really suitable for factual data like the OpenStreetMap database and there are plenty of references about it on the wiki and in the mailing list archives if you want to read up about the details.
The topic is obviously fairly controversial, as is any license change, and thus agreement hasn’t always been easy to find, which is partly why these discussions have been going on for years. In the mean time, every day a lot of people have been signing up and contributing newly, as the growth has continued to be amazing. However, that does not make the decision easier, and so it has been now decided (by the OSMF licensing working group and board with the original backing of the full OSMF members vote) to ask all new members to dual licence their contributions under CC-BY-SA and OdBL. By dual licensing contributions, it makes either outcome of the full consultation of all mapers easier to implement and thus was an important first step.
So with immediate effect the new signup page now asks people to agree to the new contributors terms that grants dual licensing of their data under CC-BY-SA and OdBL.
For existing contributors nothing changes for now and all data will remain under CC-BY-SA, as people haven’t yet had a chance to either agree or disagree with changing their contributions to OdBL and thus as every contributor retains their own copyright the license will and cannot be changed without consent of everyone.
Presumably what will happen next is for there to be a possibility for all existing contributors to OSM to express their formal opinion if they agree to relicense or not and the situation is then reassessed after a while including determining how much of the data would be able to be transfered to the new license. If there is sufficiently strong support, presumably then, and only then, there will have to be a cut of point until when people have to agree or disagree to the change.
I hope this helps to clarify the current situation and to ensure everyone who cares about this is informed.
P.S. I am not a member of the licensing working group or otherwise involved and I am only relaying information I have heard. So I can’t guarantee the accuracy of it. The official announcement can be found at http://lists.openstreetmap.org/pipermail/osmf-talk/2010-May/000881.html