
http://www.hesido.com/base.php?page=tutorials&sub=wavingflag
I've tried to achieve similar effect in Inkscape. Here's how to do this:
1. Create a new image and draw a flag (tutorial: http://tavmjong.free.fr/INKSCAPE/MANUAL/html/AShortExample.html)

2. Use calligraphy tool
and draw few black thick lines across the flag

3. Add blur (5.0) to soften the lines and group them together, then duplicate lines (Ctrl+D) and change color of new lines to white. Also move the white lines a little bit, this will create a nice shadow effect. At the end, use 'group' on all the lines.

5. Now select Object - Mask - Set


9. You should end up with something similar to this
It all looks better on flags that are not mostly white. Below is some other example I've made :]