If I remember correctly, Guardians and Defiant can talk to each other.
Mildly related, is the auction house cross faction. Same excuse the war only started a few days ago theres not reason for it to be 'segregated' besides aiding the enemy. I have a feeling that it is not. But I am curious if anybody has noticed.
EDIT: Auction house, Mail system and your friends list is single faction single server. Mail won't admit the player exists while your friends list tells you that the player exists but is on the other faction (which may or may not be nifty).
EDIT2: Not tested by me. (See post 6) You can talk cross faction.
Edited, Feb 6th 2011 10:36am by Danex317
Edited, Feb 7th 2011 12:30am by Danex317