You can register, but as I understand it you don't have to in order to buy OEM software. To be legally entitled (under the terms of the OEM licence) to buy an OEM version of Windows, you just have to buy a qualifying piece of hardware at the same time. For MCE that piece of hardware can be something as small as even a TV tuner card or an official Microsoft MCE remote control.
IIRC, for XP Pro OEM edition you just had to buy a motherboard or something with it. Of course, you are supposed to use the OEM software only with the hardware you buy with it. I don't know what the terms and officially listed bits of hardware for Vista are but I imagine you can probably find out on the MS website.
If it wasn't legal to sell OEM software in this way I'd imagine Microsoft would be quick to put a stop to it.