Map tile licenses

I’m trying to launch a map project that uses another site’s map tiles in Polymaps. Here’s links to licensing details for various tile options, along with my very cursory understanding based on my non-expert quick read of their licenses.

Google Maps API: various limits, the most important being you are required to use their Javascript library. Or so I’m told, I didn’t read the license to find the specific language.

Microsoft Bing Maps: you can use their web services with your own code, although the way the metadata service works means you may end up having to display some Microsoft branding, links etc. Education and non-profits seem to have pretty generous usage terms. Commercial use can also proceed without a license agreement, but limited to “500,000 transactions” per year. Beyond that you email them for pricing.

OpenStreetMap: It’s Creative Commons, Attribution ShareAlike. No requirements on access code or amount of traffic beyond their tile usage policy. What “ShareAlike” means for a map that uses OSM tiles as a layer is complex: see common license interpretations. The key language I noted was

If you overlay OSM data with your own data created from other sources (for example you going out there with a GPS receiver) and the layers are kept separate and independent, and the OSM layer is unchanged, then you may have created a collective work. … If you have created a collective work, then only the OSM component of the work must be subject to the OSM licence.

CloudMade. Their map tiles are derived from OSM so are similarly CC-A-SA. They may have further restrictions in their ToS.