Version: 2.7 (using KDE 4.7.2)
Using either ffmpegthumbs or mplayerthumbs results in Video previews being generated in Dolphin. There they show up, not so in Gwenview. This behaviour seems to be present from 4.7 onwards, at least reproducible on Arch and Ubuntu (11.04 with PPA and 11.10).
Steps to Reproduce:
Install ffmpegthumbs, go to video folder using dolphin and also gwenview.
Dolphin has previews, Gwenview doesn't.
Gwenview shows the same previews as Dolphin.
OS: Linux (x86_64) release 3.0-ARCH
I can confirm the bug (also on kubuntu KDE 4.7.2, 64 bit). After installation of one of the thumbnailers dolphin does display video thumbnails while gwenview does not.
*** This bug has been marked as a duplicate of bug 280879 ***