Back in the DCU of the 1980s, Swamp Thing hung with Superman, Sandman met Martian Manhunter, and John Constantine bothered anyone he damn well pleased. This changed with the advent of DC's Vertigo imprint, but things are changing once again.
According to Rich Johnston over at Bleeding Cool, Swamp Thing will be returning to the DC Universe along with those characters who debuted in DC, but — due to editorial mandate — were exiled to the Vertigo line for much the Nineties and Aughts:
I understand from speaking to a number of people that the new policy is that any character that came from the DC Universe can and will be used by the DC Universe. Prez, Black Orchid, Shade, they're all back up for grabs. [NOTE: Links added by io9.]
So while we won't see Spider Jerusalem or King Mob running around with the JLA, we could see John Constantine drop by Metropolis or Anton Arcane harangue Zatanna. Of course, this means we won't see China Miéville's Vertigo run on Swamp Thing, which has been canned to make room for Swampy's return to the main DCU.