When is newly inserted data visible to another connection?

Hi all,

If there is nothing else wrong in our test case we noticed
the following:

We have done a test with two connections to the database on
different computers.  After the first client (writer) had
inserted new data into a quite simple table, it told another
client (by TCP communication) to be ready, however, this
second client (reader) did not see the data then immediately
in the database. So we delayed the reading client from 2 to
5s to have it see all data in the table.

So, firstly: Is that a possible scenario from postgre's view
(and do we have to address it here) ?

Secondly: If yes, is there a way to determine when newly
inserted data is visible to other clients?

Remark: In case transacted write access keeps the inserted
lines together and therefore make it either visible in whole
or nothing of it -- this would not be a solution for the
question *when* it appears visible to other clients.

Thank You
 Felix