Great news: Sourcepole has extended QGIS server to offer printing functionality for web maps based on print composer.
You can test this functionality yourself on gis.uster.ch/webgis. The viewers are GeoExt-based. User can intuitively select a layout, extent, scale, rotation and resolution for printing. QGIS server will return a printable PDF.
If you want to learn more about what goes on behind the scenes, you find a good description on Sourcepole’s website.