We did do instant messaging a bit in the last game. Generally, I find the way it tends to work best is:
(1) Decide what we want to discuss ahead of time.
(2) Agree not to stray too far from these topics, even if pressed by the other team for information. (It's alarmingly easy to inadvertantly give things away in a chat with an experienced information-grabber, such as oyzar in the last game.

)
(3) Get as many people as possible involved in the chat as possible, so as little of the team is left out from the discussion.
(4) Trivial matters (such as temporary Open Borders, temporary resource trades, etc) might be decided while in the chat if there are a few people around.
(5) More serious matters that are brought up (such as long term alliances, major tech trades, etc) will need to be posted in the forum so that all members can see and express their thoughts before anything can be confirmed.
In this way, chats are not so rigid that we have to refuse to discuss anything outside our main objective, and yet not so lucid that major agreements can be made without having input from the people who didn't have the chance to be involved in the chat (perhaps due to a timezone conflict). This avoids making those people who couldn't get to the chat feel left out and powerless.