The confusion of Dell/HP/etc OEM and regular OEM, and Retail, etc versions are the reason I switched to Macs in the first place. They have 2 versions. One server, one regular. There's none of this OEM crap.
Apple and Microsoft has different models of business. Apple sells high-margin hardware like the MB, MB Pro, and iMac with OS X as "freebie." Microsoft sells software, and they've to break their software into different markets. Apple can afford to sell two versions of OS X (client and server) simply because it's the different "versions" of the hardware is the differentiating factors. Likewise, Microsoft has multiple versions of Windows XP and Vista.
Microsoft licensing:
* Vendor OEM such as Dell and HP
* OEM that you can purchase
* Retail/Upgrade (similar except upgrade requires a prior qualified license)
Apple licensing:
* No OEM, Apple is the only vendor
* Retail/Upgrade, requires a Mac