Enable HDR.
In an effort to cut a ~30 MB .bsp to 20 MB, I compiled without LDR lightmaps, which, at the time, I figured were unnecessary anyway since my graphics card seems to be able to interpolate LDR values from the HDR lightmap data. Since none of the 30 or so playtesters reported a problem, I assumed everyone else could too - obviously that's not the case.
Enabling HDR in your graphics options dialogue box should clear it up, but if your hardware doesn't support it, you're out of luck.