Mod rewrite tiene que estar acitvado (Suele venir activado en apache 2 o superior en los casos que me he encontrado hasta ahora).
Para que los ficheros .htaccess funcionen hay que añadir lo siguiente en la configuración de apache para cada directorio o virtual host correspondiente:
Options Includes
AllowOverride All
Order allow,deny
Allow from all
Para que se vea en conjunto ha quedado así en un SLES 10:
DocumentRoot "/srv/www/htdocs"
#
# Configure the DocumentRoot
#
<Directory "/srv/www/htdocs">
# Possible values for the Options directive are "None", "All",
# or any combination of:
# Indexes Includes FollowSymLinks SymLinksifOwnerMatch ExecCGI MultiViews
#
# Note that "MultiViews" must be named *explicitly* --- "Options All"
# doesn't give it to you.
#
# The Options directive is both complicated and important. Please see
# http://httpd.apache.org/docs-2.2/mod/core.html#options
# for more information.
#Options None
# AllowOverride controls what directives may be placed in .htaccess files.
# It can be "All", "None", or any combination of the keywords:
# Options FileInfo AuthConfig Limit
#AllowOverride None
# Controls who can get stuff from this server.
#Order allow,deny
#Allow from all
# 11/12/2013 - tnieto - Permitir las rewrite Rules. Se comenta todo lo de arriba
Options Includes
AllowOverride All
Order allow,deny
Allow from all
</Directory>