All my computers are pretty much always on. I wouldn't worry about them overheating, unless it's a PC, of course. Believe it or not but leaving your computer on 24/7 puts far less wear on it than turning it off and on all the time. Startup puts a lot of needless wear on the components that is far beyond that of just leaving the computer on. If you're worried about power consumption, just put the system to sleep. Sleep really does draw near no power, waking up is much faster than booting up, and doesn't put extra wear on your computer's components. As for uptime, my Mac LC III with NetBSD 1.5.2 has been up for months...my tower and iBook less only due to software installation.