I agree with all those who say the 'forehead issue' has been blown out of all proportion over the years. IMO it's clear that the TOS Klingons may not have had ridges, and the 'new' TMP look was a retcon, yes. But it was one which we were all obviously supposed to apply retroactively (ie, that was what Klingons 'always' looked like). The comment in "Trials And Tribbilations" was clearly a joke. It was the production team poking fun at the fact that they obviously had
to confront why Worf's makeup and that of the 'original' Klingons in the archive footage look so different. It speaks volumes that while the DS9 team were apparently tempted to follow up on it, they ultimately decided it was a narrative dead-end and just dropped the entire subject.
obviously had a bet each way. Their explanation was very clever, but ultimately I would hope that the makers of "Into Darkness" are smart enough to imply
the augment virus (perhaps through use of un-ridged background Klingons) without making it a plot point. After all, why shouldn't
Klingons have ridged and un-ridged? Adds a bit of diversity to their overall look.