The big problem used to be with the mailboxes file. The 2.0 release and later addressed this problem by changing the flat file to a Berkerley DB database.
Note this is for ext2fs. If you are using a newer filesystem (such as xfs, jfs, or reiserfs) the synchronous metadata issue shouldn't come up. Then again, we haven't really looked at other filesystems for linux yet. (It appears that the different filesystems support slightly different semantics, and it's not always clear what the right thing for the application to do is.)