Simsi - 5:44 pm on Dec 17, 2011 (gmt 0)
The answer probably lies in the way Google thinks. Do they think "we don't want any webmasters artificially influencing their positions" or do they think "we want to give the user the best experience". One would have thought both but with the latter being the priority.
Therefore, it makes sense that a reciprocal or "2 way" link could help, providing it does the latter. This means a 2-way link might not carry equal benefit both ways but if one side has linked to a page that is potentially authoratative, relevant and more importantly "useful" to readers of it's own page then it stands to reason that Google would see this as a good thing.
Who benefits from that most isn't obvious (there's effectively 4 connecting lines in a reciprocal link) but that's how I see it anyway.
All that said, I think link value on the whole will continue to diminish over time as algos get better at recognising quality content.