Over the past four years, I've seen GNU/Linux grow and mature in ways most people just wouldn't have expected. With corporate-backed projects such as Fedora and community distributions such as Ubuntu making such headway, it'd be wrong to say that Linux has miles and miles to go to achieve desktop adoption. From wifi support to interoperability to numerous projects on integrating FOSS drivers with the kernel itself, Free Software on the Desktop has made major headway.
However, there is a bit of a shortcoming: while distributions constantly get easier to install, remix, and redistrubute, there are only a handful of providers that even support GNU/Linux as an option. Granted, there are scores of dev teams out there for many distros that gladly dedicate their time to ensure that your machine of choice supports their distribution. It's a handy practice to have, but a problem remains in the sense that many O.E.Ms still ship Windows by default.
Another problem is that many GNU/Linux users are too tech savvy for their own good. It's been a common practice of installing a distribution on one's own hardware, and heading out to buy components for upgrade. Let's face it, it's a sensible practice for enthusiasts that like to tailor their machines to suit their own needs.
However, it doesn't change the fact that many O.E.M.s simply don't ship GNU/Linux at all. A lack of demand for machines causes manufacturers to scale back and ship what they've always shipped...Windows.
So, what distributors ARE out there to sell customized GNU/Linux machines? Below is a short list.
Dell - Dell is known for supporting Linux for quite some time, however their number of available models seems to fluctuate every now and then. Currently, Dell supports the Ubuntu distribution.
System76 - System76 has made a name for themselves by fully supporting Ubuntu across their entire product line. They have a nice lineup of laptops, although they're a little on the pricey side. Still, I've heard that they get great reviews.
Los Alamos Computers - LAC is officially supported by both the FSF and the GNU project for shipping one of the few truly free distributions, gNewSense.
HP - Last I can remember, HP shipped some machines running OpenSuSE. Anyways, HP has a dedicated R&D team for Free Software systems, and makes every effort to make their products compatible with GNU/Linux.
Lenovo - Lenovo, the old PC arm of once-computing giant IBM, has a lengthy list of machines that they sell preinstalled with various GNU/Linux distributions.
Emperor Linux - While not necessarily an OEM per se, EmperorLinux takes existing laptop and desktops and preinstalls...you guessed it! Different GNU/Linux distro of your choice.
LinuxCertified - See above.
The list goes on. Here and there, little distributors pop up with some customized laptops. GNU/Linux is growing in the sense of commercial viability, and companies are warming up to GNU/Linux Offerings.