Mobs already move in more directions than the three fixes axes.
Collision detection would probably need updating if it's possible to walk between two 45 degree sections, and perhaps things like lighting and water flow would be affected, but I wouldn't see it as a hugely disruptive/unimplementable change.
Seems the CraftBukkit team are still bug testing/developing the new version. No official release yet which means that none of the plugins have been updated yet.
I've also noticed that our permissions plugin has had development stopped, joy.
Does this mean Teh server will be operational this weekend? It's been a while and if I can distract myself from Trackmania 2's track editor for long enough I might do some MCing.