Not sure if this answer is still needing to be answered, but I ran into the same problem today working on a wordpress template and solved the problem.
What you have to do is make a wrapper before the container_12 or container_16 that is provided by the grid. You don't need html or body equaling height: 100% at all. You don't even need a min-height on the wrapper.
Make sure that your wrapper doesn't have a height or width set, and put the clearfix on container_12 or container_16 and it should expand all the way down.
I have this working on my wordpress template.
<div id="yourwrapper">
<div class="container_12 clearfix">
<div class="grid_12">
<!-- YOUR CONTENT SHOULD BE COMPLETELY WRAPPED BY THE WRAPPER -->
</div>
</div>
</div>