What processor do you have?
Pentium M and (I think) Pentium 4 processors have throttling which can reduce heat drastically when idling.
Some AMD models have this too, and there may be others.
You will need acpi, but it's not available in the 2.4 kernel. I think it was first introduced in the 2.6.2 kernel. There is a backport patch for the 2.4 kernel somewhere (google for it), but it might be easier to install a newer kernel if you're not comfortable patching your kernel and recompiling it.
Most distros now come with the 2.6 kernel, but slackware isn't one of them. You will also need a program like cpufreqd or cpudyn to handle the throttling if you install the new kernel manually(or patch your existing kernel). This will have to be installed and added to the default runlevel so they will start at boot time, but I don't remember how to do this with Slackware.
Keep in mind some laptops (like mine) suggesst you completely discharge the battery at least once a month to preserve battery life, so leaving it plugged in all the time may not be good for the battery. Removing the battery when you don't need it might help prevent this, might not..