My Dell Inspiron 1420 died recently and I got it fixed by Dell's Onsite Tech Support (which I was very satisfied with btw...). Basically my motherboard had died and the guy replaced it...he did mention that it would be advisable not to leave the laptop on for prolonged periods of time. So I was wondering does leaving the laptop on all the time really kill the motherboard?

Like all electrical devices a motherboard has a certain life, the longer it's on the more chance problems can happen, eg. electrical problems, capacitors going etc.

The biggest problem i can see with leaving a laptop on all the time is when you move it you have a very high chance of damaging your harddrive.

Heat is a big issue with laptops. Keeping them on all the time will definitely shorten the life of the laptop drastically. Especially the motherboard and it's CPU

how do i check the temperature of my laptop then? bcos most of the time it hardly feels hot at all....

Since you already had the motherboard replaced, and you were told not to leave the laptop on all the time, I would not worry about checking the temps, just use a little common sence and turn it off after using it

the life of digital equipment basically lasts for 5-7 years. the normal heat ranges for CPU's are from 15 degrees C to 70 degrees C. of course, this depends on how wide ranges are and how fast temperature changes occur. let's mention IBM TP600 which works for 7-8 years, or TP770 series.
IMHO, people should afraid development errors for specific model of laptop. and as for dell laptops, i'm only can remember inspiron 5100 series which contained many constructive errors.