No, it's not unbreakable. In cryptography, you don't throw around terms like "unbreakable" without a very solid mathmatical proof, and the only algorithm that has that is OTP. The recent obfu in question implemented an OTP, provided the files you told it to use were the same size.

Further, both RC4 and OTP are very fragile. Used incorrectly, they are completely insecure. That's why WEP is no good.