SLP - Systemvendor locked preactivated (or something).
The embedded key on the CD validates ONLY against an appropriate vendor BIOS - in any other situation, it will need to activate, and would inevitably fail to activate.
The most this could be abused (if a close enough match) would be to update earlier systems from the same vendor, or systems supplied with a different OS.
http://www.edbott.com/weblog/?feed=rss2&p=497 - some info, it MAY be possible to remake a SLP install, if you do not have one, not sure if they are suggesting that slipstreaming an older CD will break the SLP.
Ah, that was the comments, http://www.edbott.com/weblog/?p=497 this is the article. SLP is typically used by companies large enough to deal directly with MS... some known OEM SLP's are: Compaq/HP, NEC/PackardBell, Medion.
Smaller / specialist companies and shop-builds would normally be using generic OEM installs.
One thing, if you change the motherboard, or reflash with a non-Vendor BIOS, you will no longer validate under SLP.
So anyway, if SLP works, go with it, 1000's of machines are delivered pre-installed that way. Keep the COA key for when you DO make a SLP breaking update.