At my work, we purchase ~350 laptops every summer. There's 600 employees in our office, and everyone gets a new laptop every 2 years. Half the office one year, the other half the next.
We give our requirements (performance, size, weight, price) etc. to the main vendors, and then they bring their offerings to the table. The last number of years they've been flipping back and forth between HP and Dell. Every year we push them to buy Lenovo's, and every year they ignore us. They're quite a bit more pricey than the other vendors because of the higher quality of their product. Toshiba never wins, so I think last year they didn't even bother showing up. Whether or not that has anything to do with the quality, I can't say for sure. It could very well be that their warranty (or something else) isn't up to par. Seems like others have lots of good things to say about them, so I'll trust their word.
Dell and HP both make good products. Brad's comment above about Dell products being the #1 returned doesn't surprise me much.. they're "economy" laptops do seem to be made with cheaper components (I've heard that too Brad).. hence the low price. I've noticed a huge difference in quality so far with my XPS system, versus the "Latitudes" we get at my work. As others have said, what you're planning on using it for does matter alot. Another big factor is how much you're going to be physically moving it around. If it's just going to be sitting on your desk, you MAY be able to get away with lower quality stuff.. but if you're going to be taking it with you everywhere, you're going to want to look at something a bit more robust.
As far as support goes, both HP and Dell customer service are very easy to deal with. Lenovo seems to be good so far too (we did manage to get some Lenovo ultra-lites this year because both Dell and HP's ultra-lites couldn't run Vista). Can't speak for Acer's support, never dealt with them.
Hydropsyche - I support servers as well.. comparing them to laptops is apples and oranges. No offense intended. You are correct though.. HP servers are heads and shoulders above Dells..
If you want a "road warror" laptop, get the IBM Lenovo.. built like a tank. They have a cool piece of technology built in that detects movement of the laptop, as soon as you pick up the laptop, or move it in any way, it automatically parks the harddrive heads to avoid damage. Very cool feature.
I think in the end, it's the same as most things. All the "mainstream" vendor's products will be pretty comparable. Working out the little details that you're personally looking for will make the difference.