Hi caballero,
Awesome results.

Have you looked into ray marching? It's faster than ray tracing.
Ray marching is a specific algorithm, a variant on ray casting where samples are taken down a line to test for intersections or other criteria.
This is easier to implement and allows for speed optimizations via number of samples.
I'm studying pixel shaders at the moment and have the basic code running now.
This is really cool stuff.