Tom Lane wrote: 
"Paul Matthews" <plm@netspace.net.au> writes:
  
A number of points close to the common border claimed they fell into both of
the polygons.
    

How close is "close"?  There's some pretty arbitrary fuzzy-comparisons
logic in the geometric datatypes ... see FPeq() and friends.  That might
be doing it to you.

			regards, tom lane
I'll try to figure out how "relatively" close tonight, this stuff is sub-metre resolution GPS data. The attached picture shows the two polygons, the shared border, a road in this case, and the houses that think they are on both sides of the road. Houses and other features are located with latitude+longitude.

Last night I plugged in the polygon contains point code from http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html. This resolved to houses correctly. If it helps in anyway please see the attached. No use of fuzziness. Opaque yes, fuzzy no. :-)  . Use in any way you see fit.

#include "postgres.h"
#include "utils/geo_decls.h"
#include "fmgr.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

PG_FUNCTION_INFO_V1(kontains);

Datum
kontains(PG_FUNCTION_ARGS)
{
  POLYGON* polygon;
  Point*   point;
  int      isin;
 
  polygon = PG_GETARG_POLYGON_P(0);
  point   = PG_GETARG_POINT_P(1);
  isin    = contains( polygon->npts, polygon->p, point );
 
  PG_RETURN_BOOL(isin);
}

int contains( int nvert, Point* vertex, Point* test )
{
  int i, j, c = 0;
  for( i=0, j=nvert-1; i<nvert; j=i++ ) {
    if( ((vertex[i].y>test->y) != (vertex[j].y>test->y)) &&
     (test->x < (vertex[j].x-vertex[i].x) * (test->y-vertex[i].y) /
         (vertex[j].y-vertex[i].y) + vertex[i].x) )
      c = !c;
  }
  return c;
}