SELECT currval within a transaction

When inserting stuff I regularly come across the problem of wanting the
value of the primary key row of the newly inserted row. In order to get
it, I put the INSERT statement together with a SELECT currval into a
transaction, like

BEGIN WORK; INSERT INTO foo (bar) VALUES ('bla'); SELECT
currval('foo_id_seq'); COMMIT WORK;

My question is: Does this always work? My assumption obviously is that
it does, i.e. that currval always returns the current value of the
sequence as seen from within the transaction. It is conceivable
(although I don't think it would make much sense) though that currval
would return a different value if some other client did an insert right
after the INSERT in the transaction (and currval would return a higher
value). I don't believe it could be implemented that way because IMHO
it'd break the whole notion of the atomicity of a transaction. But I'd
like to be sure.

Regards, Frank