I bumped into this free plug in on page 20 of a Google search.
Its a real time displacement map viewer. For free.
Go to page 2, you'll see it.
You'll need to set the directory for it to be installed in Plug Ins, as the default during the install requires this.
Other than that (and the fact you need to close it during edits) its very cool.
And a real time saver when creating/editing displacement maps.