I think a lot of you are too paranoid about this.
I've worked on and off at Authorized Apple Service Centers for the past 10 years, and you'd be amazed how many abused computers I've seen. And, the overwhelming majority of the time, the customer refuses to acknowledge the obvious.
I've taken apart dozens of machines to find CLEAR liquid damage including obvious corrosion on circuit boards only to have the customer outright refuse any possibility that they could have been responsible. I mean, come on. People lie and deceive all the time to try to get things for free.
A lot of times people will drop (or carelessly toss around) their laptops while they are in their protective bags. No exterior physical damage will be seen because of the bag's padding, but then they'll come in complaining their hard drive or optical drive don't work (two parts with sensitive mechanical parts). While Apple (and third party service centers) will almost always cover these repairs under warranty, they really shouldn't. I think this will give Apple some protection when a customer comes in with a bad hard drive or optical drive after they've had two or three replaced already.
Someone also mentioned voiding warranty by modifying/upgrading hardware. There are very few Apple products that can be voided without physical modification to individual components. If you install a third party battery, RAM, hard drive, etc... obviously the part you installed isn't going to be covered by Apple. But that won't impact the coverage of, say, the logic board on that unit.
Apple has very, very lenient and accommodating warranty policies. I've seen all kinds of abuse covered by Apple. I've even seen them replace 4-5 year old machines with brand new units. I've seen them cover repairs 1-2 years out of warranty just to please customers. I think it is only fair Apple has the ability to protect themselves from customers that damage their hardware. There are a LOT of them.
I have a lot of complaints about how Apple does business, especially their anti-competitive practices, restrictions on using their software on third-party hardware, and closed and highly controlled software (i.e. App Store/iPhone OS)... but I can't see how this is bad. As laptops become less and less expensive and ever more common, so do incidents of customer abuse. Apple can't be expected to cover repairs of every careless consumer who spills coffee on their laptop or has it flying around on the backseat of their car everyday.