Hi,
Please first take a look at this picture composited in photoshop: http://ftpmerou.free.fr/inkscape/blur-background.png
I tried to reproduce the same effect in inkscape easilly, and i made it by using a copy of the picture, bluring it and applying a mask with a shape to simulate the effect: http://ftpmerou.free.fr/inkscape/blur-background-fakebymask.svg
It's ok now, but I thought about filter effects (background image can be used as a connection link in the filter editor) and saw this: http://www.w3.org/TR/SVG/filters.html#AccessingBackgroundImage
On http://wiki.inkscape.org/wiki/index.php/Filter_Effects there is a note about this property "There's no GUI for setting the enable-background property. This property has to be set on some parent layer of filtered object to make Background Image and Background Alpha input images work."
Does the background image support is implemented? Do you think that the same effect can be achieved with filters by applying the effect on a shape (that will blur its background content)?