Trash icon disappeared from Gmail

That button archives the message. If you go to Mailboxes>Accounts>Gmail>All Mail you can trash them from there.

Archived messages are stored in the All Mail Folder

 

It's a new setting in iOS 4. Now Gmail's default is to archive messages (by pressing that center button below the message). This can be changed in your Gmail account settings on the phone.

 

First off, all deleted gmail from the iphone (using exchange) will be sent to the All Mail folder. Doesn't matter if you move it to the trash folder or press the trash icon. Google's implementation of ActiveSync automatically archives all deleted email. There is no setting on the iphone or their webmail to change that.

The only way to ensure it goes in the trash folder is to delete it from the webmail interface. I've tried every option and setting and nothing changes this.

EDIT: here's a great reference table of Google's activesync implementation http://www.google.com/support/mobile/bin/answer.py?hl=en&answer=161771