SWTOR's creative director James Ohlen has said cross-faction communication and mailing will not be available when the game launches.
Speaking in an interview with TenTonHammer, Ohlen said that while it was open during beta testing, once the game launches this will no longer be an option.
"Currently no, you have to do it the old fashioned way through the mailbox, but only if they're the same faction," he said. "There are multiple reasons why we don't want the different factions to communicate with each other.
"We had the big argument that this isn't like Horde and Alliance, we all speak Galactic Standard so we should just allow it. So we actually did allow it for a little while. The argument against it was that, what happens is people start saying inappropriate things to the other side. That's just the way it is when you're on a different side and you gank each other, people tend to say inappropriate things.
"You won't see the other faction's chat at all, so they can't chat with you and you can't chat with them."
So basically, you can't mail and item to another character you have if it is in a different faction, and you won't be able to call Jedi names in chat or through a mail message. Bummer.
However, according to a clarification on the official forums, (thanks RockTwist) the non-cross faction chat is only for planet-wide, or world channels. So if you are beside someone in the game you can use local chat to gab as much as you like.