Dell has just released a beautiful (though a bit pricey at $1449) LINUX DEVELOPER MACHINE. Dell remains the most Linux-friendly major OEM, and has several other Linux machines available as well. Though you do have to do a search to bring them up - just enter "Ubuntu" in the search box.
Before you buy one though, compare Dell's Linux machines against those of System76. For laptops, System76 goes all the way from the very portable 14.1-inch Lemur Ultra ($659) to the 17.3-inch Bonobo Extreme ($1499). Their Sable Complete desktop ($749) looks pretty awesome too. The Sable Complete looks to compete with Apple's iMac line.
There are a lot of options nowadays, if you want Linux preinstalled on your computer. For those with just a little technical know how, it isn't that hard to get it running on a computer nowadays, dual-booting next to Windows, running in a Virtual Machine, or even running it from a USB stick (for those who don't want to soil their hard drives).