Are there open access terrain tiles with a permissive license like OSM?

Are there open access terrain pyramidal map tiles with a permissive usage license like OpenStreetMap's? I'm building aviation-related maps with Polymaps and need a good raster basemap. The Google terrain tiles are just perfect for my purposes: restrained colours, not too many features, and a clear view of terrain. Unfortunately Google does not allow use of their map tiles in any library but their own Javascript.

Alternatives I've tried.. OpenStreetMap's maps all seem to be road maps. The OpenCycleMap does include terrain, but then it has a lot of other high contrast features on top that make it unsuitable for my needs. USGS has some terrain tiles at the National Map, but they're unstyled and lack any descriptive features. I know it's possible to generate and serve my own tileset using a free datasource like SRTM, but that's an awful lot of work to design and operate.

I'm able to pay a modest cost for bandwidth; mostly I want to avoid spending the time generating my own tileset. Terrain maps seem like a common need, I'm surprised I haven't found one!

Edit since posting this question I've found a few more options. Microsoft's Bing Maps include terrain tiles; with somewhat flexible, but limited terms of service. GeoiQ/GeoCommons has the Acetate tiles which are specifically designed for my purpose.

Edit 2 this post had a small part in inspiring Mike Migurski / Stamen's terrain map based on open data. More at maps.stamen.com.

Edit 3 MapBox also now has a terrain layer. I'm unclear about the licensing, but the company is generally open source friendly.

Edit 4 MapZen now has DEM and raster terrain tiles. Again unclear about the licensing, but the company is generally open source friendly.