Longitude + latitude is not a good solution because I don't think there's any way to make the poles seamless while using linearly interpolated texture coordinates. You could try including a mirrored copy of the texture above the pole, but you'd then end up with two very different "shortest paths" between any two points near the pole.
I thought that the intention of the TEXMAP spec was to fix this by allowing normal mod-1 wrapping in both directions. However, now that I'm thinking about it I don't see how to define the projection past the 90-degree point without it becoming discontinuous.
One option, certainly, would be to use a cube map. But there's no intuitively obvious way to make a cube map out of a single PNG file, and those aren't as easy to implement or as universally supported as linear textures.
I thought that the intention of the TEXMAP spec was to fix this by allowing normal mod-1 wrapping in both directions. However, now that I'm thinking about it I don't see how to define the projection past the 90-degree point without it becoming discontinuous.
One option, certainly, would be to use a cube map. But there's no intuitively obvious way to make a cube map out of a single PNG file, and those aren't as easy to implement or as universally supported as linear textures.