Chris is dead on. If a domain is stolen it is usually a legal or contractual issue rather than a technical one. You are going to have to show that you have exclusive ownership rights to the sight and that the WHOIS information is no longer accurate. ICANN requires that the WHOIS information be accurate and they can hold the domain registrar partially responsible if it isn't. That will give you some leverage with them. Also review any of the contractual language that you had to accept when the domain was setup. Usually there will be language in there that says they will not move or alter your domain without permission from the owner. If you can show that the real owner never approved of the changes then that will also give you some leverage. Be careful here, though. If you push the point too hard then you are basically forcing the company to admit that they screwed up and allowed an invalid transfer. I read through the comments on the guy's blog, and he's already been given most of this advice. It sucks, but he is going to either have to give up the time and money to get the site back or just let the thing go.
On a side note, right in the middle of the replies you will see where an anonymous poster put the hacker's email and IP into the /b forums of 4chan. If some of those guys decide to pick up the cause for the blogger, then the hacker is in for a really bad time. They won't stop at trying to get the domain back, they'll hack his whole life. That makes me feel all warm and fuzzy. Does that make me a bad person?
CISSP, CISM, CISA, GCIH, GREM, CEH, HMFIC, KTHXBIROFLCOPTER