I was talking to Lydia recently, and she mentioned creating prototypes at CrowdCamp (which I didn't go to). But this came up when talking about something I built, saying that it wasn't a prototype, and I thought it was because it was so easy to build (and presumably it would be easy for me to change it).
However, it turns out there may be a new (to me) useful property to use in the definition of prototype, which is that it be malleable on the fly. For instance, a paper prototype of a user interface can be changed while it is being tested.
So, one upshot of this is that now the first version of PomHire is going to be more open ended, including some PiratePads (which are sortof the closest online equivalent to paper), which should allow users some leeway in how they use the system, making some things up as they go along.. to see what features should be built.