Do they sell separate Mac/PC versions, or do they claim that both images are on the same CD? (The CD itself will probably indicate).
If they are separate, then this is a different product, and a separate install. While it would be nice for them to provide a reduced-price migration path, you really would have another fully-licensed copy of the product.
Cheers.
Same product on a single set of CDs. I have Rosetta Stone French, and installed using the same discs on my iMac desktop and Dell PC laptop.