Dell is now offering the "open-source n Series desktop solution" that can be shipped without an operating system. The open-source lineup consists of 3 well-priced and affordable Dell Dimension models. The higher end model, the E520, sports a dual-core Pentium D 820 CPU while the lower end E521 and C521 desktops utilize a dual-core AMD X2 3800+ processor. The desktops come with an empty drive but Dell provides installation media for FreeDOS. They should ship them with Ubuntu discs though, as Ubuntu is well-known as the Linux beginner's distribution.
Personally, I think it is a great move for Dell; embracing the open-source community should have been an objective started long ago. Unfortunately, Dell makes it hard to find the n Series computers on their website - essentially making their open-source initiative rather ineffective to people looking to purchase a desktop without purchasing an operating system.