I agree with Jack's interpretation. "Are we on?" means "Is our plan still on?", which in turn means "Are we still going ahead with our plan?" "What's the latest?" is a way to ask "What are the latest news?" Tom is asking this question to see if anything has come up that would keep them from going on with their original plan.

Fun question. I would imagine in this case it means "are we still going to do what we planned to do?" In television, "are we on" can also mean "are we being recorded now?" but I don't think this is the situation in this case. Hope this helps.