Tracdoc,
I used Garden Wall interlocking block (with the lip on the back of each block)
They are really only supposed to be used for about 36" or so, since they are designed to lean backwards and after that many courses (4" per block), the weight would be unbalanced.
We completely burried the bottom course to prevent kickout and put the gravel behind the wall to support it forward and to prevent the clay and dirt from pushing the wall.
I got the stone from a local supplier, at just under $2/ea. It wasn't cheap, but it's permanent.
I'll post a new picture as soon as the weather clears up. We added a top ring to encircle the tree and step the ground for a better to level.
Sorry I didn't notice your post earlier, else I would've responsed sooner.
Thanks to all for the compliments.
-JC