You might want to check out maldet[0], it does a good job of finding this kind of malware. It supposedly has functionality that'll clean base64 encodes and gzipped base64 but I haven't managed to get it to work. I've posted a comment detailing the problem but the mod hasn't allowed it to be posted yet. Suffice to say, don't trust the cleaning function. I'd love for someone else to test this though.
