Gmail optimistically updates the UI in many cases, for example when starring a message or marking read/unread. Not doing so for send was a very conscious decision due to the severity of the failure cases.

That said, work has been done to ameliorate the problems and reduce the chance of data loss. Check out the "background send" lab released earlier this year : http://gmailblog.blogspot.com/2011/04/new-in-labs-background...