The 20D/30D are great cameras with noise specifications virtually
equal to the 1D mark II.  (I'll speak to the 1D II only as that is what
I have).  For me the difference that made the price worth it (it was hard
to spend $4500 on a camera, but I would do it again) are as follows.

1) 45-point autofocus sensors with ability to rapidly move the active
sensor around.

2) Speed and accuracy of the autofocus for tracking moving subjects,
and ability to quickly re-acquire focus on a subject if it slips off.
On 10D and 20D cameras, my experience is that if the focus sensor
slips off the subject (due to my error in tracking), and the background is
complex, it is difficult to require focus fast.  On birds landing, I
would usually lose the shot with the consumer cameras.  With the 1D II,
I can get focus back in an instant.  Autofocus tracking on moving
subjects is much faster on the 1D II (if you set a custom function
for higher speed, which I have enabled all the time).
Autofocus is more accurate.

3) 8+ frames per second is nice but second to the above.

4) all-weather build, and solid build (I once dropped my 1D II on
a rock from almost a meter--it bounced, but kept working perfectly).
After just returning from safari in Tanzania, I see what a beating
camera gear can take.  We had no failures in our group, including
10D, 400D and 20D cameras, but the shaking and dust of driving
off road (I really mean no roads at all here) as well as pounding
dirt roads, and the vibration would take its toll eventually.

5) Autofocus at f/8.  This is important for me for distant animals,
e.g. when using 500 mm f/4 telephoto and a 2x TC, focus is still
very good.  Even stacked 2x + 1.4x TC work (f/11).  I make nice 16x24
inch prints from 500 mm +2x and 500 mm +2x +1.4x TC images.

If I was not doing wildlife action, the 20D/30D would be all I need.
If I had only about $7K and starting over with wildlife, I would get
the 500 f/4, a 30D and then other accessories, like carbon fiber tripod.
The lens is more important than the body in my opinion in this regard.
If I had less to spend, a 300 f/4 L IS and 1.4x TC on a 30D gets
great images, is lighter and easier to carry on long trips.

I do like the microphone.  I use to carry a notepad to jot notes.
But writing notes on a pad takes more time than a quick audio
recording.  I use the audio to describe things in the scene,
identification of animals, and to record animal sounds to go with
the picture. Now I gotta figure out how to put sound on a web page
that works with all browsers.  Here is a test page with a 30 second
clip recorded by the 1D Mark II:
   http://www.clarkvision.com/features/bosque+sound
A 30 second sound clip takes 275 KBytes on the CF card, so pretty
negligible compared to the image.  I wish the 30D had a microphone.

Let's see what canon announces this coming month.  1D Mark III?

Roger
Photos at:http://www.clarkvision.com