Havent been able to find this in postgres documentation.
I know in SQL SERVER 2000 (MS) it has a log file wich stores all insert, updates and deletes that are done on the database.
In an event of multiple disk failure, you just have to put back the latest backup you have made and then use the log file (wich is usually on a diffrent disk, else the log file would be accessible either) and do the insert, update and delete that are written on the log file since your last backup.
The new database will almost be 100% restored.
So far, i havent been able to find a similair solution for postgres wich would mean that in an event of multiple disk failure... you can only restore the database since your last backup. Wich means you will get back 70-80% of the database i presume.