I accepted a similar proposal from Pericles the other day.
Next turn, all 15 AI opponents denounced me with a -16 warmonger penalty... including Pericles.
Also, you actually can't ask a player to go to joint war against someone either player is already at war with, which is incredibly stupid. This means you can never ask your allied/friended civs for help when you're under attack, and they're obviously not going to help you on their own volition, allies or not.
The whole diplomacy system needs to be tossed out the window and replaced with Civ IV's.