While I don't run mine 24/7, I do run it quite a few hours of the day as it's my prmary (and only) system. However I use a program called
i8kfangui to monitor temps and control fan speeds. Depending on the brand of your laptop, there may be something similar out there. Now, laptops have a higher temperature threshold than their desktop counterparts. So running them constantly shouldn't be a problem. Their airflow and cooling setups are carefully designed to accomodate the heat they produce and I'm sure the manufacturers stress test their system to make sure they can handle the temps. So to end my longwinded yammering

, running it all day long shouldn't be a problem provided you have proper ventilation and the fans don't fail.