Was looking to implement the product variant in magento and thought this was missing (or did I miss something here?). Presently Magento allows variants at an attribute level like color, size etc. But providing variants at product level would also help a great deal. Say a CD shop has some CDs on Tarzan and has some books on the same CD content. Here though the CD and the book are distinct products they are variants (at a product level).
There is a work around now where in we can define both in a grouped product and customer can buy only one from the grouped product. But this a vague interpretation and an inconvenient one as well.
Would like hear more on this from the community.