Sinestro was a Green Lantern that went rogue for some reason. He was trapped in some other dimension, and in that other dimension he was able to forge a GL ring, except it's constructs were yellow, since the GL weakness is yellow. That way he could kick major ass.
The worldwide release date is not yet known, because they haven't started filming yet. Jack Black has supposedly been confirmed, but it was on ain't it cool news, and I don't always trust them.
JAck black; he's a good fun guy. if he's supposed to play green latern it will work out very well despite his apperance. frankly i would mind it if he wasn't hal or kyle; instead he could be just some lucky /very creative guy that found the ring.
the "the mask" kinda thing seems suiting because with green latern's powers we would'nt want some stiff serious actor going around making the most boring of uses of the ring for a 2 hour movie when it holds so much unlimited power. this guy has to be swift fast and can think on his feet and he damned as heck better be funny. i even bet Jack black might be helping the directors with some use of the ring of his own.