My previous two tests of the 300 f/4L IS showed some peculiarities
that warranted further investigation.  Either the lens was very soft
or else it wasn't focusing properly.
So today I did a quickie test, this time with a brick wall at about a
45-degree angle from the camera.  This way I could see if perhaps it
was a focusing (or a backfocus) problem.  Once again, I used the
tripod and mirror lockup to go for the sharpest image.  The IS was
turned off since the lens was mounted on a tripod.

Here is the result:
http://www.pbase.com/bret/image/75308794/original

The red square is the center focal point used by the AF system.  That
is where proper focus should be attained.  However, as you can see,
this image has a backfocus problem.  The area to the left of the
square (the part of the wall that is farther away) is in focus while
the area to the right is not.  Further testing when I got home from
closer distances didn't show this problem.

I suspect that the IS is somehow throwing the image off.  As I
mentioned, I turned the IS off for this shot, but somehow I think the
IS system is the culprit here.  I mounted the lens to the tripod and
chose my focal point.  Then if I switch the IS on and try to re-focus
I can see the image shifting in the viewfinder.  Now when I turn it
off the original focal point may not be where it was.  So I would
guess the problem probably has something to do with how you turn the
IS off and on.  Just a guess.

Incidentally, my latest tests also included a few shots with the
400 f/5.6L which shows none of the backfocusing problems.
And even when the 300 f/4L IS focused properly it still wasn't as
sharp as the
400 f/5.6L.
So if you must have a 300mm lens, start saving for the 300 f/2.8L
which is highly acclaimed.  Otherwise, the 400 f/5.6L still rules it
in this price range.