Excellent question! I did a little research on it, and it appears that online petitions are not invalid per se, but that they are easy to falsify and are therefore often ignored. You can read a Wikipedia article on them here. I don’t think that signing the petition hurts anything. They do ask for much more than just your name in an attempt to verify that you are a real person. But if you are really wanting to help, they list several other ways to do so. Some people have done a lot of e-mailing and attempting to call Chinese leaders, for example.

I’ve just read that about Gao Zhisheng and it makes me want to help. But my question is: Are petitions without a signature valid?
I recently got a mail warning me about petitions and mails you should send on to others, saying that they were just ways to get internet addresses and that is why we get so much unwanted mail(spams)!
I’ve written to your mentioned site asking them this question.