X-Plane 940 now supports normal maps on OBJ models (both scenery and airplane). I'll get more formal docs up once the rest of my office is moved and unpacked but here's the details for now:
The normal maps are in "blender" format see here. The alpha channel is optional; if it is present, it serves to modulate the level* of specularity. Opaque means full specularity, transparent means none. You can use this feature to make some parts of an object shiny and some dull on a per-pixel level.
Shininess level is modulated by both ATTR_shiny_rat and the alpha channel, so you need ATTR_shiny_rat 1.0 and an opaque alpha channel (or no alpha channel) to see full specularity.
Normal maps are only available for objects and only appear if pixel shaders are on and per-pixel lighting is enabled.
Normal maps should be PNG format, not DDS - they will not be texture compressed because S3TC compression tends to kill them. (There are some modern formats for normal map compression supported by the newer cards but we don't use them yet.)
* Specular level: most serious 3-d programs let you control both the specular exponent, which controls how "tight" the specular hilights are, and the specular level, which controls how bright they are. X-Plane only lets you control specular level; if specular hilights exist, they are always as the maximum exponent for the sharpest specular hilights.