Question 1 : is it possible to have such a replication configuration with the streaming replication of PG9 (cascaded replication) ?
Nope, as far as I have tested pg only has 1 master and can have a number of slaves, so having 2 masters is not possible.
Question 2 : All the procedures I have seen describing a switchover between 2 PG servers require to copy (or rsync) the database from the new master (old slave) to the new slave (old master).
Is it possible to do switchover between sites (between PGMaster1 and PGMaster2) whithout copying all the database from the new PG master to the new PG slave ?
If it is not possible yet, shall it be possible in future releases ?
Nope this is not possible, pg requires to have an updates basebackup at the slave, this is because if you loose a lot of streamed wal files it is impossible for the slave to catch up and have consistent data.