Does the BOINC client have to run all the time or can it only engage as a screensaver. If so, is it ensured to use minimal CPU time except when the screensaver engages (as in the old OS-X seti@home client)?

It pretty much has to run all the time... or you can stop it and start it as desired. As M2 comes to bear, I think it'll be an option to run it as a screensaver, but for OS X, right now, there are no graphics at all, anywhere. Sorry. :-(

The graphics are on their way, though. Rumor has it there's a developer working on the SETI@Home graphics engine for Linux and Mac and I know Rom is hard at work on the core client GUI to do controlling and administrative stuff. :-)

Rob

BOINC does run at a low priority, so it will use only the CPU cycles not used by other programs. If you have adequate memory, it should not be a problem to leave it running in the background all the time. I do that on my iBook 600, and have experienced no problems.