Maybe You should check on your AMD OCL APP SDK. And your graphic driver. On a friends machine some OpenCL optimizations forced Opencl Compiler to Use CPU due they are not (yet) implemented in GPU code. amd_printf for example can behave like that. Are there any other devices shown for usage? I am asking that because maybe guiminer messed up to enumerate right through the opencl devices since all amd cpu's are recognized and usuable devices.
u may pm me if u still having troubles.