PgAdmin 1.9+ has a simple Plug-In architecture which makes it relatively simple to introduce new plugins. It is pretty much all controlled by the file plugins.ini. In that file you can register any executable you want in there. If you want the executable to get passed database configuration settings, there is an option for that and you just have to build your executable to accept commandline switches.
You can download the windows version from http://postgis.net/windows_downloads. If you just want it without the PostGIS 1.5 binaries -- just download the one labeled PostGIS ESRI Shapefile GUI.
To enable said plugin, do the following:
; ;PostGIS shp2pgsql-gui (Windows): ; Title=PostGIS Shapefile and DBF loader Command="$$PGBINDIR\postgisgui\shp2pgsql-gui.exe" -h "$$HOSTNAME" -p $$PORT -U "$$USERNAME" -d "$$DATABASE" -W "$$PASSWORD" Description=Open a PostGIS ESRI Shapefile or Plain dbf loader console to the current database. KeyFile=$$PGBINDIR\postgisgui\shp2pgsql-gui.exe Platform=windows ServerType=postgresql Database=Yes SetPassword=Yes
NOTE: That for geography your data most already be in WGS 84 long lat (4326) which is the only spatial ref supported for geography since transform support is not currently built into the PostGIS loaders. For data you get from Tiger and Census boundary files(although they are in NAD 83 long lat (4269), you can get away with lying and calling it 4326 for most use cases, since they are almost identical in spheroid,datum etc to the WGS 84 one)