I don't think Rondo will leave in free agency. Even if he wanted to, I think the Celtics would match any contract he was given next summer. That means he won't be able to leave as an unrestricted free agency until June 2011.
At this point, I think it's too early to know who the replacement options will be. However, in June 2011 the team is potentially looking at a lot of cap space, if they need to add a replacement.