I stumbled across the Wikipedia page for them:
http://en.wikipedia.org/wiki/Fusion_tree
And I read the class notes pdfs linked at the bottom, but it gets hand-wavy about the data structure itself and goes into a lot of detail about the sketch(x) function. I think part of my confusion is that the papers are trying to be very general, and I would like a specific example to visualize.
Is this data structure appropriate for storing data based on arbitrary 32 or 64 bit integer keys? How does it differ from a B-tree? There is one section that says it's basically a B-tree with a branching factor B = (lg n)^(1/5). For a fully populated tree with 32 bit keys, B would be 2. Does this just become a binary tree? Is this data structure intended to use much longer bit-strings as keys?
My Googling didn't turn up anything terribly useful, but I would welcome any good links on the topic. This is really just a passing curiosity, so I haven't been willing to pay for the PDFs at portal.acm.org yet.