Which led me to the thought, maybe therion should warn, but continue if it finds an co-ordinate outside the area covered by the cs selected?
I'll second that. A couple of years ago I was trying to work out why north became south! It was a cs out of range error due to a mistyped digit.
It is not trivial for projected coordinate systems. It would be necassary to specify the range for each CS used. Valid ranges for some (not all) EPSG codes are listed at http://spatialreference.org/. Perhaps the realistic implementation would be to let the user specify valid range for any CS he defines (CS definition is not supported yet, but planned). Then if you want to be sure with your coordinates, just define custom CS by copying EPSG definition and add valid range from http://spatialreference.org/. Martin