This page discusses usage of PostgreSQL schemas for prefixes. "Normal" prefixes can be used in the same way as in MySQL, so they won't be discussed here.
PostgreSQL has something called 'schemas' (http://www.postgresql.org/docs/current/static/ddl-schemas.html). They can be very handy sometimes, but if you don't know what they are, you probably don't actually need them and can stop reading here.
Schemas can be used as prefixes within Drupal. That is, with a multisite setup, each site can reside in its own schema, and shared tables can reside in a "shared" schema (or even in the public schema).
There is one annoyance: the upgrade will fail. This is unfortunate, but nothing can be done as the "normal" (not schema) and schema prefixes are just incompatibile. If you are interested in the details, please see http://drupal.org/node/40034.
But, don't worry. This can be easily fixed by changing the update script (update.php and updates.inc) a bit. The problem lies in the CREATE [UNIQUE] INDEX
and ALTER TABLE ... DROP/ADD CONSTRAINT
statements. When schema prefixes are used, queries like this are executed:
CREATE INDEX prefix.search_total_word_idx ON prefix.search_total(word)
ALTER TABLE prefix.boxes DROP CONSTRAINT prefix.boxes_title_key
ALTER TABLE test.contact ADD CONSTRAINT test.contact_category_key UNIQUE (category)