Strongly reminds me of the X11 logo (one of my favourite designs).
The X.Org Foundation might need to do the same for their X logo even though it's a bit different (and I'm not sure if it's trademarked in the first place).
IANAL, but you do need to defend a trademark in order to maintain it after all.
Of course fighting to protect it would probably be costly and financially risky even if they did have a case.