It appears that you can add events while composing messages -- almost as if the message is the event. The ability to add events through Gmail will be quite useful by eliminating some potentially annoying double entry.
Another interesting screenshot shows a link bar at the top. When this was first discovered a couple months ago, the option for "Calendar" said "CL2" -- maybe they have decided against the whole "CL2" thing and went back to "Google Calendar"? I think the link bar is a great idea as it will help Google promote their other services and make it easier for current users to flip between them.