No materials or lights when rendering with GPU in cycles

When I turn on GPU rendering in cycles none of the materials in my scene are rendered, but the materials work correctly on CPU rendering. My card is ATI Radeon HD 5870 with 1GB memory. Any idea what causes this? None of the mesh lights are working either. It seems to render on environment light.

I would really like to use GPU rendering since doing 50 samples takes only a few seconds while on CPU 10 samples takes about 10 seconds and the result is still very grainy.