There was a time when it was practically impossible to buy a computer with GNU/Linux pre installed. If you wanted to run Linux, you had to install it yourselves. Almost all PC manufacturers were shipping PCs with Windows OS. And if the buyer did not want Windows, he still had to pay the full price of the machine which included the cost of Windows OS.
But with Linux getting more user friendly as well as its improved out of the box support for various hardware devices, more and more people are opting to run GNU/Linux and its market share is increasing at a steady albeit glacial rate.
In my country for instance, it is possible to buy an assembled computer with Linux installed - something which was not possible or very rare a couple of years back. But the most significant aspect is that Acer, the Taiwanese PC manufacturer is selling its range of laptops with Linux pre installed and a Linux laptop from Acer is significantly cheaper than its Windows Laptop of the same make.
Lxer has a very interesting post which lists all the companies which offer PCs with Linux pre installed which could be useful for anyone interested in buying a Linux laptop or desktop.