Impressive work :) If you manage to get plaintext from just cyphertext, be sure to spread it around; I'd imagine a lot of people would be interested in it.
It doesn't quite do that yet. Given a section of known plaintext of about 50 characters it can find the original key used (or at least enough of the key to then turn the problem into a mono-alphabetic cipher).
It could use things like a configurable key and interactive enciphering command line, and I would be happy to pull such changes.
Edit updated link with improved comments about the rotation part of the cipher