Not sure if this is getting rolled out everywhere but Larry spotted this last night and we're seeing it in a lot of SERPs:
Pretty interesting. Really I think it's just more emphasis on domain authority. Not unlike site links and the search within search bar, a triple listing allows Google to crowd the SERPs with the single destination they've deemed sufficiently authoritative.
The criteria for attaining a double listing  had been that you would need to push both pages into the cluster of ten a searcher was viewing. I haven't done a lot of testing but I find it hard to believe this is still the case, given the frequency with which I'm seeing the triple listing.
What do you think? Is this an extension of the Brand/Vince update ? Just a test? How aggresively are you seeing it in your SERPs (if at all)?