Cassette spins freely in both directions

[andrew_s]

So how is the freehub itself attached to the wheel? If I wanted to replace it rather than keep it clean would I need the special tool?

Shimano freehub bodies attach with a hollow bolt that takes a 10mm allen key poked down where the axle would normally be.

Some of the more recent hubs (eg XT) use a thicker aluminium axle and presumably need a bigger allen key.
There have been reports that the freehubs on these are more fragile than older freehubs. Presumably this is because the bigger axle means smaller bearings which don't last (just like Octalink and Isis bottom brackets).