Mike wrote:
What is the usual treatment for hard clay soil, should soil this bad
be dug up and replaced?

What I started with: http://img20.imageshack.us/my.php?image=soil1.jpg

As others have said you will need to break it up with much organic matter.
And as drainage may be a problem doing this in raised beds can be an
advantage. Also you will get some help breaking it up by adding gypsum
depending on the clay minerals present. Broadly gypsum makes smectitic
clays clump but not kaolinic, right now I cannot find the test used to tell
what you have but there is such a thing if you are prepared to go looking.

As for the decision to remove it and import soil that depends on the
situation. If you have a small area, want quick results and have the money
to spend then replace it. You are looking at the price of new soil, quite a
bit of earth work and somewhere to dump the old soil.

If going this route take care not to build a pond. Assuming you have a
solid layer of clay and dig a big flat hole in it which you fill with nice
well-draining soil. If you have heavy rain your plot has only as far as the
edges of the hole to drain to then it fills up with water as the clay around
it is impervious. Then nearly all your plants die. You need to plan and
build drainage for the whole area as part of the replacement.

David