Hi there.
Nice module. The very good reason it will be needed is to split node creation in steps and set condition, and only allow user to go to the next if some conditions are ok.

Example: user entered value in cck field which I want to be distinct above all nodes ever. I do a Computed Field and check in data base if the new value is really new. If not - protect doing the next step.

I actually cant find how can I stop the next step for certain condition...?