Well, first attempt (on an Ubuntu 12.04.1 LTS 8800GT) only generated Compute Errors. I've yet to dig in to why. I suspended BU on that machine to get a chance to peek at the logs. I'll try an ATI card shortly.
EDIT: Easy. Seems "executables" need the Xecute bit set. Running my first task on the 8800 right now.
EDIT2: Got the ATI going! Plan class doesn't matter, but it does help to say in the app_info.xml that the GPU <type> required is an ATI one. Running my first task on a 5770 now.