Comment 8 for bug 951816

There's another important use case in which this feature would be very helpful: some print drivers (e.g. CUPS) convert PDF to PostScrip (PS) before printing. As PS does not support transparency, the whole page is rasterized which looks poorly. This is especially problematic when printing documents with text and inserted graphics. A transparency flattening option when exporting to PDF could prevent this problem. This problem does not seem to be too uncommon as the following links show:

https://askubuntu.com/questions/128595/flattening-pdf-transparency
http://superuser.com/questions/550155/inkscape-transparency-when-exporting-pdf
http://mathematica.stackexchange.com/questions/29120/how-do-i-flatten-transparency-on-a-graphics-for-conversion-to-eps-or-similar

If the last link can be believed, Inkscape would be the onyl free graphics software capable of doing this. Apparently only Adobe products can do it so far.