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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](http://www.csse.monash.edu.au/~jwb/cgi-
bin/wwwjdic.cgi?1MUE%E3%81%A3%E3%81%91) does that, hence the question.
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particles questions sentence-final-particles
Why is it that the っけ suffix  can only be attached to a question-sentence
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_
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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.
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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.
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