I've personally had very good experiences with Dell machines in terms of reliability, particularly laptops (my own Dell Studio 1558, which is currently running Linux Mint 14, Ubuntu 12.04, and Windows 7 in a triple-boot and still runs incredibly, even though I've had it for two years and it was a refurb when I got it). My old Dell Dimension desktop (from the early 2000's or so) is still chugging along pretty well with Debian Squeeze, and my employer uses Dells for all the workstations. Based on my own experiences, I'd therefore recommend Dell if that's something you're willing to consider.
I've heard great things about System76, too, so that might be worth considering; the price point's a bit higher, but you're getting quite a bit of bang for your buck (especially that Bonobo Extreme, even at minimal configuration), and it comes with Ubuntu preinstalled, so it's going to be Linux-friendly out-of-the-box (at least, you'd think). I'm considering them for my next laptop purchase, now that I'm doing things like development and gaming under Linux and can use the souped-up hardware.
