Super Meat Boy Forever is playable at the show, but there's no set launch date just yet. "When it's done," is the timeline for now. See a Vine of Super Meat Boy Forever straight from the PAX Prime 2014 show floor below.
Earlier in August, Team Meat teased something called "A Voyeur for September," which conveniently anagrams to "Super Meat Boy Forever." That's about the only resemblance the teaser has to this new game, since the accompanying video was a live-action stealth(?) thing.
Update: For clarity's sake, Super Meat Boy Forever is an auto-runner, rather than an endless runner as we first described. Either way, that Vine looks like razorblade-lined barrels of fun.