A user named ReklissAbandon, employed by Naughty Dog, accidentally revealed that cross game voice chat will be included in the next firmware update for Playstation 3. His original post on the official Playstation boards read as follows:
"I think they wanted people to have fun and ejoy the show since all dead players can talk to each other no matter what team you are on. But I will suggest it anyway. Not to mention that the next PS3 firmware update is going to allow cross game chat. So if players use that instead of in game communication then they would also be able to communicate enemies positions."
The post has since been removed, but hopefully this long awaited feature will be made available in the near future.