Gmail Labs has introduced a new button Send & Archive .
When you reply to a mail, you get the option to Archive the conversation at the click of a single button.
The big question is, How many will actually find this useful!?
Having separate Send and Archive buttons gives you the flexibility to not archive a particular conversation in case you deem it fit to stay in the inbox. However, the Send & Archive button would take away this option completely thus forcing you to archive conversations every time you send a reply.
I guess this will be one of those features which will stay as a Labs feature forever!
Also please note that the undo feature has yet to be implemented for this button.