A friend of ours on the mailing list, Lisa Yimm, informed me of a website she started for rendering folks in search of high-quality HDR panoramas to use for image-based lighting:
There seem to be some really nice scenes in there.
Meanwhile, I'm busy at work on a new panorama stitching feature for standard and HDR images in Photosphere. Look for that in the next week or two. Unfortunately, it won't be able to create full spherical panos like the one on Lisa's site, due to the warping and stretching that would require.
-Greg