Currently Being ModeratedAug 6, 2011 3:40 PM (in response to Tyler Routson)
Go to Mail -> Preferences, click Accounts and select your GMail account, then click the Advanced tab. Under "Keep copies of messages for offline viewing", choose "Don't keep copies of any messages."
Currently Being ModeratedAug 6, 2011 3:57 PM (in response to Tyler Routson)
That has always been visible for any GMail account, since at least Mac OS X 10.5 (aka Leopard), as far as I recall. I'm not sure what you did to not have that visible.
One way that you can get rid of some of those mailboxes is to set Mail to use them. For example, click the triangle to expand it and then select the Drafts mailbox. Then choose Mailbox -> Use This Mailbox For -> Drafts. It'll disappear, but now Mail will use that mailbox as your Drafts folder for GMail. Same with Sent and Trash.
Others, you can log on to your GMail account, click the "gear" icon and choose Mail Settings, then the Labels tab, and uncheck the Show In IMAP box next to any mailboxes you want to hide from Mail.