The license for the OS talks about three things being "Apple-branded": systems, computers, and hardware. In all three cases, Apple failed to provide a concrete definition of what constitutes a "system", "computer", or "hardware".
Looking at these, "hardware" is likely to be narrowly interpreted. The term "computer" is more broad, but I think is also narrow enough in scope to not be an issue.
However, if we interpret "system" with a regular meaning, it seems likely to include the display, keyboard, mouse, power cord, and any other peripherals necessary to be useful to a particular end user's needs. This interpretation brings up some issues, and is rarely found in the EULA for Mountain Lion 10.8.2. The issue specifically is that it prohibits attaching any non-Apple device in creating a system. So a non-Apple power cord, for example, would be an EULA violation for the operating system. While this is within Apple's rights in an EULA (just as they could demand you only use solar to power it), I can see a court removing the requirement that a system be composed entirely of Apple-branded components.
Companies selling Apple computers with non-Apple hardware installed are not hard to find. Modbook will likely do something very similar to OWC in how they handle sales
. Just as with Axiotron, I suspect that the end-user buys the computer from Apple but has it shipped to Modbook's facility, where Modbook then strips out the motherboard, installs it in their case, and ships it to you. End users can do whatever they want to their systems, even if an EULA says otherwise. They just won't have a warranty anymore.