In master/slave mode(external replication tool such as Slony-I and Streaming replication is responsible for duplicating database), now() is correctly handled as long as those replication tools work correctly.
In replication mode, pgpool-II is responsible for replication. Since pgpool-II is a statement based replication tool, all DML/DCL statement are sent to all PostgreSQL servers. If this applies to statements using now(), this would be a problem.
Actually pgpool-II solves the problem by using "query rewriting technique". Consider following INSERT statement:
INSERT INTO t1 VALUES(1, now());
Pgpool-II issues "SELECT now()" to master server to retrieve the timestamp constant. Then rewrite the INSERT by using the constant. Actual rewritten query will be something like this:
INSERT INTO "t1" VALUES (1,"pg_catalog"."timestamptz"('2012-10-30 09:35:43.910192+09'::text));
Therefore each PostgreSQL servers use exactly same timestamp value.
What if now() is using as the table default value? I mean:
CREATE TABLE t1 (int i, timestamp t default now());
and "INSERT INTO t1 VALUES(1)" issued? pgpool-II is smart enough to realize that now() is used as a default value and complements it:
INSERT INTO "t1" VALUES (1,'2012-10-30 09:51:22.880077+09');
Note that not only now() but other time/date functions including CURRENT_TIMESTAMP, CURRENT_DATE and CURRENT_TIME are processed like now() as well.
BTW, in the conference I also heard that "random() cannot be handled by pgpool-II". This is true. I think we could handle random() exactly same way as now(). Question is, if it's worth the trouble. If there's enough demand to handle random(), pgpool-II developers will gladly attack the problem.