I've had luck with the Dell Latitude line, and have had three of them so far. The Dell 630 is a sound notebook built around the Core 2 for reasonable money. They hold up well, and components are easy enough to find. You can find them just about everywhere and don't have to spend much either.
In my opinion, they don't respond as well to my typing as my Thinkpads, but nothing else has that I've used enough to comment on.
I was just about to reply and say something similar until I read this Post. So, all I can say is that I completely agree. I have a little bit of a side-business refurbishing and repairing computers, mostly for other teachers and their families. Although I do much of my work with ThinkPads, because of all the characteristics that have been cited already and that I believe in them, however a lot of my clientele asks for somethings that's a little less expensive. So referring back to experience from my IT days when I often bought Dell Latitudes when I didn't have the budget for ThinkPads, I found the D620 and D630 to a great fit.
The specs are very decent: with Core Duo or C2D, DDR2 for RAM as well as SATA hard drives making upgrades and replacements far cheaper than the older D600/610. They sold in such great numbers, that they are very easy to find as well as cheap to buy. I've often been able to get them in the $150 range. The main thing to watch out for is to be certain you get the original Dell A/C adapter, or you'll get an error message and the expense of having to replace generic that many eBay sellers try to include. It's also worthwhile to keep an eye on the "E" series as well, since they are reaching that magic 2-3 year "end of lease" term time-frame.
But, definitely stay in the "professional" lines of whoever you go with, since the service is completely different. Although, not as nice as IBM/Lenovo support, the Dell service has been fine. I'd rate it a little above HP, and FAR above others that I've dealt with such as Acer, Gateway and Toshiba.