In ordered jounalling mode of ext3 data is always written to disk prior to the metadata (journal data). This ensures data consistency in most cases. However the performance of this option is not that well.
In writeback journalling mode the metadata might get comitted to the journal before the data is actually written to the disk. This might lead to data corruption in some cases (e.g. power-off before the data could be written to disk but the journal data already has been commited). But on the other hand performance of this option is considerably better.
The YAST2 installer allows to set the writeback journalling mode but not in the / filesystem. If one specifies the data=writeback option in /etc/fstab the / filesystem is mounted in ordered journalling mode although. To get around this one can use the tune2fs tools with the option '-o journal_data_writeback'. This sets the default mount option for the filesystem with which it gets mounted in writeback journalling mode. The same applies to the 'journal' journalling mode.

So my suggestion for the YAST2 installer is to call' tune2fs -o journal_data_writeback' for the respective device if a user selects the writeback journalling mode on ext3 for the / filesystem. I've tested this on three different machines without any problems.