I was pleasantly surprised to find very much enlarged filter support in QGIS-Dev (1.9.0_Master).
One application would be a WFS-Filter which combines $geometry and Intersect in order to load only the data from WFS which would be visible in the actual extent.
Something like this: intersects( $geometry, geomFromWKT( 'LINESTRING(765145.88 6274561.22, 776031.67 6284189.52)' ) )
But the above mentioned statement is not a valid expression in the "Expression string builder".
What is the correct way to phrase it?