Mark,
The easiest thing to do would be to use a varchar column for the postal code. This makes sense to me because postal codes can by non-numeric (Canadian postal codes are a good example). Additionally this would result in a blank rather than a 0.
Another point, if you just call Model.new this will not persist to the database, thus a nil field should remain nil until you actually persist and the data is loaded from the database.
Finally, I highly recommend using migrations if you are not already. You can save yourself lots of time in the long run if you get used to them.
HTH.
V/r Anthony Eden
