Hahaha, I found that interview at TV Squad while fruitlessly searching for Marshall pics for my "you've been lawyered" icon, and I just found it again and even the URL is spoilery, but here is the relevant paragraph:

Bays says that he and Thomas know where the show is going and what the plan is. Which means that fans can rest easy; the writers haven't forgotten about Barney's unwise selection of slap punishment: five slaps at any time of Marshall's choosing, without warning, for infinity. "Our goal in this show is to never leave things unresolved. Eventually all the questions will be answered every open storylines will be concluded. And, yes, we will see more slaps."