Why is it that the っけ suffix / け particle (indicating that the speaker is trying to recall some information) can only be attached to a sentence when the sentence itself is a question-sentence?
Because all 10 example usages i see in WWWJDIC does that, hence the question.
Although these sentences are nearly always translated as questions, the (admittedly informal and possibly colloquial) usage of っけ followed by か suggests that there is a difference at least on some level. In my experience, "questions" formed via the っけ particle are often rhetorical -- but just as often, they are interpreted as a request for information.
Basically though, the answer is yes, use of っけ to indicate an attempt to recall information pretty much always turns the sentence into a question.
Edit: Although I believe that this is implicit in my original answer above, I though I would more explicitly answer your question post-edit.
It is not so much that っけ can only be used with questions, but rather that it turns sentences it is used with into a question. For example, consider the following:
- あなたの名前は何ですか。
- あなたの名前は何でしたっけ。 (warning: this is probably a bit rude.)
This is an example of っけ being used on a sentence that is a question-form (i.e. it uses an interrogative, in this case 何); but it doesn't have to be, as in the following examples.
- ここはオハイオ州です。
- ここはオハイオ州でしたっけ。
In these sentences, っけ is indicating that the speaker cannot recall what state he or she is in; by saying this, he or she is implicitly requesting the listener supply that information, which is to say that っけ is turning the sentence into a question
だっけ denotes a question of the form "(proposed fact) ... is this so?", where (proposed fact) is a fact that the questioner once knew but has since forgotten. It is not rhetorical, at least not in the sense that the questioner (now) knows the answer. The questioner is no longer sure of the answer, and is seeking confirmation.
外国のレストランでのチップって 10% だっけ？ : I forget; is the tip at foreign restaurants 10%?
10% はショボいだろ。 : 10 % is a bit stingy.
Since nobody has offered what I consider to be the most straightforward translation for 'っけ', here I go:
When added at the end of a question (usually a rhetoric one), the closest equivalent in English would be to add "... again?" at the end. E.g:
なんですか？ → What is it?
なんだっけ → What is it again?
どこですか？ → Where is it?
どこだっけ → Where is it again?
etc. etc.
(indeed, marking that you are trying to recall something you once knew or were supposed to know)
Just like this "again" in English, 'っけ' is rather colloquial (but not rude).
Update: I thought the connection would be obvious enough, but apparently not for everybody. So let me spell it out: just like saying "where is it again?" or "is it here again?" (with proper raising intonation in both case), you usually need to be asking a question (rhetoric or otherwise) for 'っけ' to make sense.
