I thought I read in the manual of WP that it doesn't need/use .htaccess for SEF url's, but that obvious was a mistake
My lay-out however is as follows:
/var/www <-- serendipity resides here
/var/www/other
/var/www/anotherdir
(etc).
In this lay-out I found out I *do* have to add the .htaccess because as soon as I turned on the mod_rewrite option in S9y, every url outside the root started to redirect to S9y...