Rather than discuss something I don't know or understand like many of the poster's above (who seem to be leveraging former experiences with prior OSes) I will state my experience and you can accept it for it is.
I purchased an ACER M5 with a pre-loaded OEM version of windows 8, similarly there was no affixed sticker stating the product key. I removed the original hard drive and upgraded to a new SSD. No efforts towards imaging or maintaining of the original HHD were performed. I purchased a system builders DVD for windows 8 pro to install onto the new, entirely blank SSD. During the installation, I was never asked to input a product key. My final istallation was a copy of windows 8 (non pro). Futhermore, a check on the last 5 digits of the product key applied on my installation reflected a different product key than the one shipped with the system builder DVD.
Speculation: the OEM product key was stored elsewhere on the machine, detected, and employed rather than asking the user to supply a product key. I've read elsewhere that the key is maintained in the BIOS for new OEMs from major PC manufactures to simplify reporting to Microsoft.
Personal: I build a new box every two years so the fact that my product key was never requested doesn't impact me; my purchased personal use license for W8 pro will be used eventually.
1This will only work IF and ONLY IF you are using the machine the OEM license came on. – Ramhound – 2012-11-01T13:49:39.863
Yes, that was the point of the whole question :) So you're saying it will work, although the most up-voted answer says it won't? – Peter Jaric – 2012-11-01T20:00:49.187
@PeterJaric: depends what you mean. Windows won't automatically detect the license, but provided you type the activation key in from the OEM sticker it will work. You don't need to buy an additional license. Note that OEMs are required to put a sticker with a license key on the machine even though you won't need it if you use the preinstalled image. – Harry Johnston – 2012-11-01T20:34:53.807
1In my particular case, the laptop model (ASUS UX32VD-R4002H) doesn't come with a sticker (mine doesn't and I've seen one other report to the same effect). Is this something new with Windows 8, maybe? – Peter Jaric – 2012-11-05T12:34:14.633
1
There are tools to fetch the product key Windows 8 was installed with from the registry.
– Gene – 2012-11-22T08:10:47.683Did you check in the battery compartment? The stickers are often placed under the battery to prevent wear. – Harry Johnston – 2012-11-22T20:54:13.610
You can't take out the battery on the UX32VD without unscrewing the cover, so my guess is that isn't there. I recently did open it to replace the SSD and RAM, but I didn't look for the sticker then. – Peter Jaric – 2012-11-23T11:59:54.883