libpq is thread-safe as of PostgreSQL 7.0, so long as no two threads attempt to manipulate the same PGconn object at the same time. In particular, you cannot issue concurrent queries from different threads through the same connection object. (If you need to run concurrent queries, start up multiple connections.)
PGresult objects are read-only after creation, and so can be passed around freely between threads.
Libpq clients using the crypt encryption method rely on the crypt() operating system function, which is often not thread-safe. It is better to use MD5 encryption, which is thread-safe on all platforms.