different coordinates system

[Gregor Mosheh]

> I am building a mapamundi mapserver application.
> I think (I am not sure) that I need to use geographic coordinate system
> to show a map of the world.
> The problem is that I have some postgis information of my region with
> epsg=23031.

Are you using a PROJECTION block in your layer and also in your MAP
object? You would need one for the map itself (the output projection,
latlong?) and one for the layer (the input projection for that particular
layer).
Something like this:

MAP
  # the map's target projection
  PROJECTION
    "init=epsg:4326"
  END

  # for each layer, specify its input projection
  # and MS will automagically reproject on the fly
  LAYER
    NAME "mydata"
    PROJECTION
      "init=epsg:23031"
    END
  END
END