Laptops are the only thing I will purchase an extended warranty for, because they are so hard to repair.
Almost all laptop parts except for the hard disk are specialized. You can't buy a motherboard from one place, a case from another, a LCD screen from a third place, and screw it all together. In fact, just the replacement cost of a LCD screen usually is almost the cost of the laptop itself.
Sometimes you don't get what you pay for. I have a $1200 laptop that I bought in 2003 which is completely unusable because the fans make a piercing keening noise. Desoldering the fans from the mainboard would cost more than its worth to make the laptop useful. However, a laptop I picked up for $500-$600 in December of last year is decently built without issues (other than the Nvidia chipset epic fail on a lot of laptop makes.)
My advice is that if you are on the road with the laptop a lot (as opposed to the laptop being mainly sitting on a table for its life), an inexpensive one will last you 2-3 years before something big fails, something that is so relatively expensive that you might be better off just replacing the thing. This doesn't include batteries.
If you buy a more expensive laptop, it should last you a lot longer, assuming the normal wear and tear of the road. Macbooks and Macbook pros seem to be very roadworthy machines, and business grade IBMs, HPs, and Dells are not bad either. However, don't assume that buying an expensive consumer model means that it will last longer than a cheap consumer one -- usually you are paying for more bells and whistles (dedicated video being the big one.)