OEM software is specific licensing to allow computer manufacturers to install Windows on their hardware. There are two important parts to OEM licensing - you aren't supposed to install an OEM copy of Windows on anything other than what that copy was installed on (obviously you can upgrade stuff, but a Dell OEM license isn't legally valid on an HP, etc.), and OEM licensing requires that the licensee support that copy of Windows. Microsoft provides no support or warranty for OEM-licensed; that's the responsibility of the OEM.
It has nothing to do with being "in cahoots" in some illegitimate way - OEM licensing is a way for OEMs to license Microsoft products cheaply, and it's a way for Microsoft to offload support to an OEM (who is going to be better suited to handle technical problems with the OS that are related to their own hardware, anyhow).
It sounds like Windows Server 2008 Foundation is only available through OEM licensing, which means that it only comes preinstalled on a server that you purchase with that OS.
You may be able to install this version of Server 2k8 on other hardware, but it will be in violation of your EULA. This is true of any OEM copy of Windows, which is why, for instance, Dell Windows installation disks won't boot on non-Dell hardware.
If you want a copy of Server 2k8 to install on a Mac Mini, you're going to have to buy retail or buy one of the OEM options from Newegg, as long as you're aware that OEM copies of Microsoft operating systems do not come with support from Microsoft.
Also, you can read the full terms of the OEM license here:
http://oem.microsoft.com/downloads/P...se_English.pdf
Processor versions. For Server products, you may not preinstall or distribute a processor version of the Software on a Customer
System that includes more processors than the highest number of processors listed on the Pack.
From what I gathered on Microsoft's website, Server Foundation is specifically and exclusively for single-core processors. Installing it (or attempting to) on processors with more than one core is a violation of the OEM license.