A dilemma is when you do not know which way to turn.
I didn't know Geo. Bush had so much interest in language or so many votes.
Are you people kidding? Most voted that "is when" is ok for formal writing/ I can't believe that! They pounded that into our heads when I was in school. The definition of a word has nothing to do with time, so how could "is when" be appropriate? Come on people!!!
That is your particular school. I think it's likely that it was not using common rules.
At the core, when is referring to a specific, but unknown, instance of the situation, and the situation is a dilemma, so it just barely works.
Not knowing which way to turn is not a dilemma as long as one can easily turn back, no matter when.