Very sad and unfortunate. For me he was one of the pioneers of PC98 game music. A few days ago when I saw this thread appear on top of the board I already felt something bad had happened.
His friend and colleague Hideki Higuchi made an entry in his blog
http://tynwald.jp/news/index.php?eid=104
It seems one of Umemoto's last involvements was music production for the novel game Tenshi no Hane wo Fumanaide, released on July 29.
I will miss him.