How to selectively replicate a PostgreSQL database
Postgres built-in replication is based on log shipping, and as far as I know does not support this (you're shipping the Postgres transaction log from the master to the slaves, so everything - including deletes - gets passed along. You may be able to approximate it by keeping all the transaction logs (forever) so you can roll back to a specific point in time, but that's not a particularly efficient technique.

You can probably rig up something like this with trigger-based replication mechanisms like Slony, but a better solution might be a standard mirror plus the table auditing (tablelog) project which basically creates a historical log of what happened to a table (in a shadow schema) so you can see what was done when, and by whom.
This is best combined with 9.x-style master/slave replication, where you can read from the slaves to poke around in the audit history.