Well, what do computer scientists do, when they are bored? They toy around with cryptography.
For some reason I didn’t get video_entropyd to run (it would throw v4l errors and segfault), but I desperately need entropy. Now what I came up with is a quick python script which does essentially the same thing, but with much more dependencies and high-level scripting languages. Also I just grab video frames from the TV-card with pygame.camera, extract the entropy and write it to a fifo. All the communication with the kernel then does rngd from the rng-tools. It picks up the bits, checks if they are really random and only then puts them into the kernel. All in all I have a solution, which (according to rngd) generates about 80MiBit/s of entropy from a good old Bt878 receiver. Im quite satisfied :)

update: it turned out, that was only a number which resulted from reading and writing in chunks. The long run performance is 8kbit/s of entropy.

download source code

Python Video Entropy Daemon v0.0.1 (works on my machine)
(c) 2010 by Kai Dietrich

Inspired by video_entropyd by Folker Vanheusden,
The main part actually is just a python version of Folkers code.
http://www.vanheusden.com/ved/

This software is Licensed under the
GNU General Public License 2.0 or later.

System Requirements:
--------------------
Python 2.6
PyGame 1.9.1
a video4linux device
optional: rng-tools / rngd

What it does:
-------------
Pyved records frames from a video4linux device, extracts the noise/entropy
and writes it to a fifo.

Pyved opens the first video4linux device it finds and records frames (720x576, RGB).
If it finds the kernel entropy pool to be empty it starts extracting noise
from two successive frames. Every uncorrellated change in one of the three color
channels is considered to be a bit of physical randomness and written to
the fifo "entropy.fifo". You can then attach rngd from the rnd-tools to this
fifo (rngd -f -r entropy.fifo). rngd will test the noise with a FIPS 140-2 test for it's
statistical randomness and delivers the bits to the kernel entropy buffer.

How fast is it?
---------------
On a Pinnacle Bt878 analogue TV card, tuned to a really bad channel,
rngd reports the following speeds (entropy bits per second):

stats: HRNG source speed: (min=1.330; avg=1.783; max=4.657)Gibits/s
stats: FIPS tests speed: (min=70.382; avg=88.529; max=89.969)Mibits/s

this is frickin fast, compared to all those commercial devices

It does not work, what can I do?
--------------------------------
a) read the code (it's not that much)
b) fix the code
c) tune your tv-card with a tuner application to some channel before starting pyved

How can I enhance the code?
---------------------------
Just do it. If you like this tool, you can just set up a project somewhere
and start collecting improvements. For me this was just some fire-and-forget
single-task code.