I'll assume you've all read the "latest" issues of the Ultimates. If you haven't, screw you
Anyway, we see in the last issue that Giantman is apparently helping the "Anti-Ultimates" with his Vision and Ultron robots.
I know, as do we all, that he was expelled from the Ultimates because of spousal abuse, and rightly so. But still, does anyone really believe that he sold out?
It seems to me that he's playing the "Anti-Ultimates" for fools and waits for the opportune moment to kick them right in the scrotum.