According to EU CM Wryxian, Blizzard developers are looking into hopefully adding a quest line that will allow warlocks to use green fire in Mists of Pandaria.
Remember that the key word here is "hope." Even the slightest possibility of green fire in MoP should have most warlocks salivating. This is especially true since Chaos Bolt, which is one of only two green fire spells currently in the game, has been removed from MoP talent calculators. Given that green fire has been one of the most requested features of the class for years and that MoP is an expansion bringing to us loads of vanity choices in glyphs and challenge mode sets, this seems an obvious way to go.