#839 tracks the idea of adding custom, fast transforms for certain simple and common srs's. But, for this to work we would also need to detect if the srs is 'geographic' for proper calculation of scale denominators (zoom levels). Really all mapnik cares about is whether to convert from degrees to meters: http://trac.mapnik.org/browser/trunk/src/scale_denominator.cpp#L38 but the meaning basically if it is "unprojected" long/lat or platte carre: http://mathworld.wolfram.com/EquirectangularProjection.html
[springmeyer] proj4 does this by checking for the keyword 'longlat' or 'latlong' in a proj4 literal string, and for epsg codes, expands the code into a proj4 literal string before checking.
done in f8b1520 and #1703