You probably need to purge the postgresql-client package as well, so that your shell will find /usr/local/pgsql/bin/psql which should be the one that you installed from source.
I believe there is a "backports" repository with up-to-date versions you can just apt-get though. Any Debianites like to go on the record as to how to set that up?
-- Richard Huxton Archonet Ltd
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