The majority of the ontologies however are what is know as Directed Acyclic Graphs, which basically means that any given Class can have multiple parents. This multiple parenting is one of the things that make ontologies so powerful, but unfortunately it obscures a hyperbolic visualization a little.
The TouchGraph model ('Force-directed graph layout'), seems to have no problems with Directed Acyclic Graphs, and the results are therefore pretty cool and dynamic (see again: Demo).
I must definately credit Mathieu 'P01' HENRI for his wonderful examples & scripts on how to create diagonal lines by manipulating HTML only, and Sean McCullough for that illuminating blog post on force-directed graph layouts, complete with code examples.
Conclusion: Can be improved a lot, it's an experiment, but a fun one with great looking results. One downside: Loading lot's of nodes turns the application into a slideshow instead, but if we keep it reasonable, it seems to render quite fast.