With Windows Server 2008 there were two types of SKU for each edition: the normal one and the “without Hyper-V” SKU. Any “without Hyper-V” SKU could never run Hyper-V. At the start it was said to be $28 cheaper than the normal SKU. Later I heard it was the same price. I know in the SPLA world there was a small price difference.
R2 does not have a “without Hyper-V” SKU. It confused people and could lead them to buy a license and never be able to run Hyper-V with that license/install. We in Ireland advised people to not buy it if they were in fact the same price.
The question remains now … can you upgrade from Windows Server 2008 x64 Without Hyper-V to Windows Server 2008 R2? I’ve not tested it and not seen anything definitive yet.