folding and SETI are two totally different things that just happen to use the same research architecture: distributed computing.
a computer downloads a set of basically raw data and runs a pre-defined set of operations and calculations on that data that typically take quite a while to process.
then the computer processes in it's "spare time" and you basically see very, very, very little slow-down in almost all situations. If you run the console version of folding@home or SETI on anything after an early G3 (or early P3 if you're a PC person), you probably won't even know it is on.
After your computer has processed a work unit, then it sends back the results.
the net result is a virtual supercomputer that can basically handle more number crunching than anything else on the planet.
"folding" refers to the type of research that folding@home does...they study how proteins in our bodies "fold," which is a barely-understood and crucial-for-research process. It's one of the weakest links in the "how life works: exhaustive edition" archive.
We're helping to fill in the blanks by donating wasted computer time.
iBook, to answer your question, yes, it's fine to run it all the time.
if you have stability problems on your machine, though, you may not want to run it. i know several people have trouble with iBooks...if you're not one of them, then the only reason i can think of NOT to fold is because you don't want your battery life to suffer...which i suppose it could, because there is a constant "full" load on your processor when F@H is on..
so if you spend a lot of time not "plugged in" then you may want to avoid using the console version of F@H, because if you're uncomfortable controlling stuff in the console, you would find it a bit difficult to turn F@H on and off.
and to anwer your question, wdlove, I have a dual 1.8 G5 and two little old 300mhz G3s...needless to say, the G5 is pretty much the only one doing anything worth mentioning...the other two are basically headless and folding all the time...the G5 is kept quite busy in photoshop, etc by yours truly...