With SoCG leaving ACM we have been told that SIGACT and ACM conferences cannot have any formal arrangements at all with the new conference or do anything official that might support it. (This decision was made at the very top of ACM and not by the staff.) This rules out any joint sessions... It also means that SIGACT will have to end our participation in this formal coordinating group.While I await more information and do not want to rush to judgment, if this description is true, I find it unacceptable behavior on the part of the ACM. Their job is to promote computer science. (The ACM home page begins with: "ACM, the world’s largest educational and scientific computing society, delivers resources that advance computing as a science and a profession.") Their job is not to try to monopolize and control the computer science conference market.
I encourage everyone to read Jeff's post (here's the link again). Obviously, this is an issue for Computational Geometry, and an issue for Theory. But I would say it's an issue for all ACM members, and in particular those that publish research at ACM conferences. It would be nice if, should it become necessary, members from other areas in computer science voice their displeasure with ACM's actions in this case.