Yes, those optimizations I was talking about: having database server store transaction log in high speed solid state disks and consider it done while background thread will update data in slower disks...
There is no reason to wait for fsync in slow disks to guarantee consistency... If database server crashes, then it just need to "redo" log transactions from fast disk into slower data storage and database server is ready to go (I think this is Sybase/MS SQL strategy for years).
Using a nonvolatile write cache for pg_xlog is certainly possible and often done with PostgreSQL. It is not important that the nonvolatile write cache is fronting for SSD, fronting for HDD is fine as the write cache turns the xlog into pure sequential writes and HDD should not have a problem keeping up.
