Using boinc-client as a daemon, as standard installation does, causes GPU to be not detected (I'm using proprietary ATI driver from rpmfusion for this). If I copy the /var/lib/boinc directory under my home directory and run boinc-client from here the GPU is detected and works, so it's not a boinc problem, but maybe some SElinux/directory/user permission (?). I already tried to add 'boinc' user to 'video' group, no effect.
*** This bug has been marked as a duplicate of bug 879776 ***