It is in /usr/local/Cellar/postgresql/8.4.0/bin/, but `which initdb` returns "/usr/local/bin/initdb". The /usr/local/bin one is a symlink to the /usr/local/Cellar/postgresql/8.4.0/bin one.
Running "/usr/local/Cellar/postgresql/8.4.0/bin/initdb -D /usr/local/Cellar/postgresql/8.4.0/defaultdb" yields the same results.