Postgresql - Streaming replication - STONITH
well, you cannot switch back easily.
what you can do is to use the old backup method:

SELECT pg_start_backup('whatever');
rsync the data dir
SELECT pg_stop_backup();


this is the best way as not all the data has to be transported over the network.
i definitely recommend this replacement for pg_basebackup because it is the only RELIABLE way to get things back in order (nobody knows for sure what happens before the crash).
sure, there might be other tricks out there but i would strongly vote for this method.