I had to sign back in when I saw this, kinda been a ghost for a while. My first post and thread here worked on this very subject: http://www.gangsterbb.net/threads/ubbthreads.php?ubb=showflat&Main=31789&Number=812802

My thought there was that for Tessio to truly succeed, he has to have a master plan to take out Barzini as well at the same time as Michael, or Tessio is getting whacked by Barzini after he's done the dirty work of getting rid of Neri, Clemenza, Rocco and possibly some others in the old Corleone guard. That's all assuming that any one of those three don't put the pieces together for themselves and take out Tessio first. No easy task to set for himself there.

The scenario I had envisioned was for Michael to ride with Tessio to the 'meet' and along the way, they pass through a certain toll booth. The car gets gunned, 'shades of Sonny' style and Tessio manages to 'survive the attack', but wasn't able to save Michael. Hence why Michael coming in a different car 'screws up all his arrangements'. Tom was presumably going with them since he stated that he couldn't go with him in the novel and movie, so I tend to think Tessio may have been planning to take Tom out as well. On the heels of that, Tessio marshals the Corleone resources and uses the killing of Michael as a reason to launch an attack on Barzini. Barzini would be presumably waiting at the 'meet', and so would be a sitting duck for Tessio's attack. If Tessio succeeds in taking out Michael and Barzini without drawing suspicion to his own hand in it, he can take over the Corleone and Barzini holdings in one master stroke. That also gives him a good possibility of being able to secure the loyalty of Neri, Clemenza and Rocco as well (or at least, not have them out to get him).

Tom, like Michael, knew that whoever approached Michael with a peace deal had turned traitor, and Tessio would've had to take him out, lest he blow the whistle on it all.