NaN is a primitive Number value, and the Ember.typeOf doc says 'number' represents “Number primitive or Number object”.
The current behaviour means Ember.typeOf(parseInt(someValue)) always yields the same result, although I don’t have an example where this would be useful.
So, I second your question. Ember.typeOf returns 'null' and 'undefined' where appropriate, so I think it would be consistent for Ember.typeOf(NaN) === 'NaN', since this method gives finer distinctions to the types it detects.