Dell has sporadic quality control problems as a result of the way they build systems. Sometimes they get a bad batch of parts and it finds its way into systems manufactured during specific time periods.
And money is almost always the reason in answer to your question. I'm sure Dell is quite aware of the problem, and I'm sure they probably have had someone run some numbers to see what it would cost them if they did nothing, compared to what it would cost them to issue a recall on those parts. The fact that they haven't done anything yet means that they figure it will cost them more to issue a recall than to just lose those customers. Because if Dell issues a recall, then what are they going to do with all of these AC adapters they have sitting around in some warehouse? They're just taking up space, they can't sell them, testing each one individually would be cost prohibitive, and disposing of them would also cost money.
Anyway, like I said, Dell has sporadic quality control issues, but on the whole they're pretty good. How much that will change once they start doing like HP, and start using asian slave labor to build their systems on the cheap, I can't say. HP/Compaq was never known for producing quality systems, so it's not like it was damaging for their reputation when they started using asian slave labor when Dell's Wal-Mart style business model was eating everyone's lunch in the PC business. Amusingly, it finally came back around to bite Dell in the ****. Now I can't wait for the same to happen to Wal-Mart.
Lenovo is a Chinese (I believe) company that bought IBM's computer business a few years ago. Just like Hitachi bought IBM's hard drive business a few years before that. So far as I know, Lenovo has done a remarkable job of keeping up the quality levels that IBM was known for. You will pay a bit more, and maybe not get as many optional features, but Lenovo laptops should take a pretty considerable amount of abuse before giving up the ghost. Toshiba would not be as good as Lenovo, but they are still a good solid choice.
In the end though, always remember that quality costs money. You buy something cheap, odds are it's made of cheap parts. Companies like Dell, Lenovo, Toshiba, etc, are in business to make money. You don't make money by buying top range products and selling them at cheap knockoff prices. So keep that in mind. Everyone along the retail chain wants to make a profit. So if you buy something from a big box retailer, they are going to want to make a profit on the sale, the company that made the laptop will want to make a profit, along with every company that supplied any number of parts for that laptop. So if you factor all that in, and you see some laptop that's under $500, you've got to start wondering exactly where the corners were cut to get that price.
And one final tip... Buy direct, and oh so rarely does anyone who works at a Best Buy or Circuit City type store have any clue what they're talking about. Radio Shack seems to do a pretty good job training their employees, but I consider them an exception. You go in looking for a 10ohm resistor, they know what it is, and exactly where to find them. You ask someone at Best Buy or Circuit City something simple, like if they have any y-splitters for headphones, you tend to get a blank stare like a deer in headlights. If you're real lucky, they'll take you to some aisle where they have ipod accessories. Put another way, some of them are so incredibly inept, it's enough to make you question the validity of evolution, because you'd swear there is no way these people should have been able to survive the natural selection process for this long short of divine intervention.