I have just set up Gmail on my iPhone4 using the Exchange/IMAP method (m.google.com).
Everything works great - the only problem I'm having is that when I send a new mail from my iPhone to myself, I get the new mail notification twice - once for the new mail in the inbox (as expected) but then a 2nd notification chime for the sent item.
I want to keep my Sent Items in sync between my iPhone and Gmail, however, I'd rather not get the Sent Item chime/notification (and have to go in and read it to clear it from my 'unread mails').
I also noticed similar behaviour if I send a mail via the Gmail website (again, to myself), my mail.app (Mac OS) will show both the new/unread mail in both my Inbox and Sent Items - same behaviour.
Any ideas how to sort this?