Planets does support multiple monitors in that it runs on all screens; this is unlike some screen savers that simply black out all but the primary screen. Unfortunately, while we'd love to have all screens run as one big screen, it is not feasible from a technical standpoint. Specifically, Apple's screen saver engine runs each screen instance as a separate process (program). Thus, to run all screens as one requires each screen's instance to send its state information to every other screen's instance. In the case of Planets, this would require sending information about the mass, location, and vector of every object between each screen instance after every frame of animation, resulting in a potentially very large amount of data being sent around. Since Planets performs all of its calculations in real-time, this additional burden could overload slower processors and buses. Even on faster machines that can more easily handle the load, most users don't want their screen savers consuming more processor time than necessary because it slows down background processes. As for running on dual-headed PowerBooks, it's not surprising that performance is poor. The slowness is not related to running independent scenarios on each screen--as I discussed above, doing so would actually make things slower--but rather because PBs do not contain sufficient video RAM to handle OpenGL on both displays simultaneously. As such, you will notice a similar slow-down on any screen saver or game that tries to do full-screen OpenGL on both screens.