GaiaX, a company that sells community Net services to ISP's and portals filed for a trademark for "Blog" on March 6 of this year. They issued a press release on June 28 saying that they would reserve the right use the word blog in products and services, but would allow people to use it freely in writing. I think some people have doubts as to whether they will be granted the trademark, but stranger things have happened in Japan. In the past, I think someone trademarked "groupware". I think GaiaX also has a trademarks in some categories on "Avatar."
Anyway, I'm glad SixApart and MovableType don't include the word "blog" in the tradename. ;-P