Here's a few more details (and confirmation that there is a legal action pending) from Godzilla historian Ed Godzisewski (right) that was posted at the Monster Zero message board:
Yes, there is a legal case ongoing in Japan. Has been for a while. I have heard about the case from Ryuji for a couple years. The family has preferred to just quietly pursue the case the way it should be done, in court. It all stems from the Japanese system that gives rights to writers (not directors).
The first ruling is due in February.
Looks like we won't have to wait too long.