Graywolf performed an interesting test on his site for outbound linking effect on improving rankings.
While not a big enough test to draw concrete conclusions, it certainly offers some insight to how linking works and the possible effects on rankings.
One of his “findings” was:
What does this mean from an SEO standpoint, well I wouldn’t be linking out from pages that you want to rank for a particular term. If you have to link out, use anything other than your desired keyword as the anchor text. However I wouldn’t start creating websites that are linking dead end’s, it would seem to easy to spot something like that programatically. I would however eliminate or keep to a minimum the number of outbound links on your “money pages”. Instead try placing the outbound links on secondary pages.
Performing a similar test on a larger scale could be an interesting exercise. Also, tests can be run on the kinds of sites linked to (eg. authority, relevancy, etc.). Knowing the true effects of outbound linking could possibly translate to higher rankings.