AFAIK this is actually incredibly common. The example that springs to my mind is people writing matrix libraries that manipulate the code to "compile" it to use more efficient algorithms. People do this kind of thing in C++, such as in Blitz++, but you end up having to pull heroics with the type system (and end up with some pretty abused syntax at times) to accomplish what is much easier in a homoiconic language like Lisp.
