Since in my Jaunty (very hacked) install I realized I hadn't thumbnails of html files (nor I could remind if I had those the time I upgraded from Intrepid or even before) I started looking at this issue.

It wasn't too difficult. I had some hint from even if the guy who wrote that couldn't solve his problem.

Well, if you also lack them, just fire gconf-editor and search the key named "desktop/gnome/thumbnailers/text@html".
I found that its command started with "gnome-web-photo" yadda yadda yadda thus omitting the path to the executable. Changing it into "/usr/bin/gnome-web-photo" was enough.

Then, "killall nautilus" or better restart the gnome session. Done