ScepticMatt: They announced it as E3 sale in the CDProjekt Summer Conference.
Then it begs the question, why doesn't the GOG version have the same offer, even though CDP stands to gain the most from the site? (Other sites like Steam gets a cut) I'm guessing any announcement made by CDP is merely for publicity's sake for Steam's service - after all:
1) A sale on Steam or otherwise is still a sale for CDPR, even though they may lose a bit from the sale, but potentially gain more from those Steam-only users refraining because of the patch.
2) It is their title after all, people who had issues with the size of Steam's patch complains and floods CDP's thewitcher.com site too. So it's only reasonable they try to appease potential Steam buyers since Steam's issue on the patch is also CDP's issue in a sense.
This may also account for why of all the services (Digital and Major Retail Chains), GOG which has the smallest patch file did not get the discount. Since GOG.com and hence CDP gets 100% of the purchase price, even though the same principle applies (lower cost per item coupled with higher number of buyers = greater revenue), CPR does not think that the price difference will make much of a difference since potential GOG users will not be turned away by the patch problem, unlike the other two platforms. (Both of which comes with DRM, even though retail can now be patched to be SecuROM less, the stigma remains. There are users on Steam forums which now refuses to buy anything with SecuROM even if it's only a disc check. They just hate its presence in the first place.)