There is a quick way to get an 8.4.3 installation that failed working. Open Terminal.app and run the following:
cd /Library/PostgreSQL8/bin
sudo ln -s postgres postmaster
cd ../lib
sudo ln -s libecpg.6.1.dylib libecpg.6.dylib
sudo ln -s libecpg.6.1.dylib libecpg.dylib
sudo ln -s libecpg_compat.3.1.dylib libecpg_compat.3.dylib
sudo ln -s libecpg_compat.3.1.dylib libecpg_compat.dylib
sudo ln -s libpgtypes.3.1.dylib libpgtypes.3.dylib
sudo ln -s libpgtypes.3.1.dylib libpgtypes.dylib
sudo ln -s libpq.5.2.dylib libpq.5.dylib
sudo ln -s libpq.5.2.dylib libpq.dylib
exit
That will recreate the missing files from the distro.