The Inkscape team has just released version 0.45, and it includes initial support for SVG filters. The first one implemented is Gaussian Blur. Raquel Ravanini was quick to try it in KMahjongg tiles. Notice that we already have shadows, but these are currently made of several gradients put together. They look good, on most cases, but the seams are difficult to manage. This screenshot shows the new tile using a simple Gaussian Blur filter (right), and our current approach (left).

In the case of this particular tileset the difference in rendering is not significative, but having support for the SVG filter in Qt/KDE would make the life of the artist easier, and the filesize smaller as well. Sometimes it is difficult to achieve good results at larger resolutions using multiple gradients. Zoom in and notice the small gaps in the shadows apparent in this screenshot of the classic tileset:

(Before you comment on it, of course this is just an example, we know about the gaps, there is still work going on on the Classic tileset and some adjustments are going to be made before the final version to correct them.)

Unfortunately, at this time QSVGRenderer apparently does not support SVG filters. Rendering this svg file with Qt 4.2 gives us the following result

I must mention that I really did not expect it to work yet, of course. Firefox and other SVG viewers do not work as well. Support for filters is expected in Firefox 3.0, however.
It appears that SVG filters will become increasingly popular in the near future due to Inkscape, as they solve real design issues and help keep the filesizes manageable. It is probably not a trivial feature to implement, but hopefully future versions of QSVGRenderer will include support for this technology. Maybe in time for a KDE 4.1? Maybe if we bribe Zach, or clone him?