Depends. For the ds, i would think so just because so many people now own flashcards and would rather just download it.
Ofcourse there's still people who purchase it, but i would guess many more people would rather not.
For console or pc games, their sales are fine, just because you can't play games online if you download them instead of buying them, and for most games for me anyways, i think online is the main reason i'd get it. For example, i have almost 30 hours of Killzone 2 online and 30min. into the story mode.. I don't play on beating the story for a while either.