How do I get archive to work in Mail.app in Lion?
If you're okay using Mail's delete function as archive, then simply go into Mail > Preferences > Accounts > Gmail > Mailbox Behaviors and deselect both of these options:

[ ] Move deleted messages to the Trash mailbox

[ ] Store deleted messages on the server

Screenshot:



Then when you delete a message in Mail, you'll be archiving it.

Reference here:

https://mail.google.com/support/bin/answer.py?hl=en&amp;answer=78892