Having paid fairly close attention to Matt Cutts' comments about 301 redirects since 2003 or so, here's my understanding, although I can't point to exact quotes...
A 301 redirect is like a page with one link on it. (unless it links to a second 301)
Like PageRank passing in normal pages, a certain amount is not passed on. In PageRank version 1, 85% of the PR value could be passed while 15% is "lost".
My understanding is a 301 is subject to this exact same PR loss situation. So in theory, if the dampening has not changed, a 301 is 85% as good as a direct link.