On my N900 I did:

apt-get install postgresql-client-8.3

and got:

Setting up postgresql-client-8.3 (8.3.3-0maemo1) ...
/var/lib/dpkg/info/postgresql-client-8.3.postinst: .: line 5: can't open /usr/share/postgresql-common/maintscripts-functions
dpkg: error processing postgresql-client-8.3 (--configure):
subprocess post-installation script returned error exit status 2
dpkg: dependency problems prevent configuration of postgresql-client:
postgresql-client depends on postgresql-client-8.3; however:
Package postgresql-client-8.3 is not configured yet.
dpkg: error processing postgresql-client (--configure):
dependency problems - leaving unconfigured
Errors were encountered while processing:
postgresql-client-8.3
postgresql-client
E: Sub-process /usr/bin/dpkg returned an error code (1)

But indeed I am able to connect to my remote database (which is actually 8.4 on Ubuntu 10.04, redirecting to a different port) using, in the N900 Terminal, this command-line:

/usr/lib/postgresql/8.3/bin/psql -h 72.72.72.72 -p 7272 -U scott

Beware: some phone operators allow you to use only a few selected ports (80/http, 443/https...) filtering out non-common ports.