According to dnscoop.com:

"The estimated value of http://diablo3.com is: $2,289,330"

I think that, even if Blizzard took them to court, they could make a case to at least get part of that. They could argue that their domain has actual value and that they aren't trying to hold the domain hostage.

However, the guy was a moron and gave it away so this is all pointless anyway.