I come to this rather late, but I would like to offer an alternative.

On "sparring partner": I think your suspicions are probably sound. To me there are two issues here. First, is it an apt metaphor, and second, is it actually used in English-speaking business contexts?

My answers, briefly, would be yes, to some extent, to the first question, and no, on the whole, to the second. A sparring partner is someone who helps to train a boxer for a fight by probing his weaknesses and testing his skills in a mock-combat. I am surprised by Tony's statement that the notion of combat is totally out of place in a business context. This seems to me manifestly untrue. If you are pitching for business, trying to get hard-nosed investors to part with their money, you can expect a rough ride. Your presentation will be probed for weaknesses, often quite aggressively. So someone who prepares you by subjecting you in advance to tough questioning, playing devil's advocate, trying to shoot you down so you learn how to deal with it before you "get in the ring" for real, is by no means alien to a business context, it seems to me.

(Just in parentheses, this reminds me of when I was applying for academic posts, years ago, and had some interview training: someone gave me a grilling, brutally probing the weaknesses in my answers, to teach me how to handle it in a real interview. This sort of thing is common in a number of fields and I think "sparring partner" might include this kind of idea.)

I don't think a sounding board is quite the same thing. A sounding board is someone to give you an opinion, to try out ideas on, but without any suggestion of combative questioning designed to prepare you for dealing with a negative reception. It's a different role.

But yes, when you look, most uses of "sparring partner" in business contexts do seem to originate from companies with German, Dutch, Scandinavian or other foreign-language connections. It is sometimes found in bona fide English-language websites, but usually as a metaphor, sometimes actually in inverted commas, rather than an actual expression.

I think perhaps the notion of a business coach and of business coaching might be what we're looking for. It doesn't necessarily imply the "mock combat" kind of approach, but it does include the idea of training in a quasi-sporting sense, and the coach is explicitly distinguished from the mentor and the consultant:

"Coaching can benefit executives and succession planning candidates by providing a “safe” confidant to bounce ideas off of, brainstorm, discuss very sensitive issues, navigate troubling situations, and act as a ‘sparring partner’ for ideas and strategies before announcing or acting on them.
It is “lonely at the top,” and many very high level executives find that having a coach in the background is truly a ‘secret weapon’ and provides a competitive edge over those who try to ‘go it alone.’ Coaching is a very trusting relationship that entails goal setting and skills development to ultimately lead an executive to a better future. [...]
Can’t a Mentor, Consultant or Seminar Achieve the Same Results?
Company Mentors are effective with aiding employees in understanding the politics and culture of the company. They can advise on internal ways; however, they are not always ‘safe’ when discussing difficult situations within the company. A coach is able to discuss life-outside-of-the-firm and how to become well rounded with skills that go beyond your current function or company culture.
Consulting work is focused on specific business issues, usually surrounding process, policy, or procedures. Coaching is focused on YOU."
http://www.wolfmotivation.com/blog/coaching-the-secret-of-su...

Note the use of "sparring partner" in inverted commas in paragraph 1 of this quotation: it's the right idea, but not the standard term.

See also, for example: http://en.wikipedia.org/wiki/Coaching#Business_coaching

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Note added at 2 days18 hrs (2013-09-12 10:41:05 GMT) Post-grading
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I'm very glad! I must say I found "Alison's "thought/thinking partner" an interesting approach.