After some convincing from some folks on IRC and a re-read of the CTs I've ticked the accept button.

My head is in knots at the moment trying to comprehend all this legalese with logic, but I'll try to explain my line of thought.

Basically by uploading to the OSM API, I am publishing works. Sometimes I am the copyright holder of such work, and sometimes I am uploading work that someone else is the copyright holder to. I ensure that if I am not the copyright holder to the work, that the work is indeed compatible with the current OSM license -- CC-BY-SA. If I am the copyright holder, then I grant everyone a license to do anything that copyright would forbid, i.e. I license it CC0. That is my intent. But I can't guarantee this. A lot of my work needs to be CC-BY-SA because it is derived from nearmap, and the other bunch is derivative works of existing OSM data, which also must be CC-BY-SA.

If I interpret the CTs as I give you the rights to do anything with the data I upload, so much as I can, but don't guarantee such, rather than I guarantee that you have the rights to do anything with the work I upload, then I believe I can tick the agree to the CTs button.

anyway... I'll have to think about this some more later.