I would believe that it is better to leave it on than turn it on and off all the time...
Many of you know that turning off the computer manualy (without going to start>shutdown) by pressing the button on ur case or wutever is not good, though you might not kno why... It's because of the power in the components... when you press the button on the case, all the power in your components (video card, sound card, motherboard, cpu, RAM, etc. etc...) is unexpextedly gone. I have noticed that computers that are turned off this way frequently begin to lose the sharpness in their performance... me and my brother both got the same computer a long time ago (800 Mhz, 128mb ram, 4mb integrated video, HORRIBLE) and he didn't seem to care about its performance and he would turn it on and off however he liked... my computer (which i have given to my mom) is still alive and kicking like it was when we got them two years ago... my brothers computer is barely holding onto its last thread... constant problems, blue screens, and unpredictable freezing.
so back to my main point... just as the power is unexpectedly drained from the components, the surge of power coming INTO your components when you turn your computer ON is also not the best thing to do to your machine... tho many of you have turned on your computer hundreds, no THOUSANDS, of times and may not have noticed anything it still isn't good for the computer.
I leave my desktop computer on for about a week or so before i end up needing to turn it off for one reason or another (windows updating, installing programs, or just to "refresh" my system). My desktop runs PERFECT still. I've got an 1800+ with 512mb ddr, an ATI AIW 8500dv and an 80gb hard drive and 48x cd burner (want to get a LITTLE bragging in here haha) and I have recently overclocked everything with the fsb (i kno not the best thing to do but i don't have a thorough bred processor) and I have my cpu running as a 2000+ now and it runs even better than wen i first put it together (the most exciting moments in my life haha jk).
so yes I would believe that leaving your laptop on constantly would not be harmful to your system, tho I would make sure to do the battery maintenance thing that everybody is talking about. I think I have heard around here or maybe sumwhere else that when your batteries are being charged that the compuer aoutmatically drains them and THEN recharges them... but I think this might have been from sumwhere else for sumthing else. I dunno, I'm getting all my information mixed up!!
hope this is sumthing new for most of you to hear! hope this helps!