What is the safe+fast way to move all Postgresql data to another machine? I need to copy all data from one Postgresql 9.2.4 server to another (both on similar hardware, Ubuntu 12.04). Is it safe to shut down the source Postgres server, rsync the data directory to the new server and start up both servers? Do I need to copy the pg_xlog directory as well? Or is it ok to leave that out since the source server will be shut down cleanly (service postgresql stop)?

I have approx 275G of data so don't want to backup and restore as that will take ages.
No, the pg_xlog directory can't be omitted even if the server has been properly shut down before copying the data directory.

If you try, you should see that the target server will fail to start, complaining that it can't find a checkpoint record.