Earlier last week, we talked about, the IMAP feature for Gmail. The feature has been officially announced on Gmail blog. Gmail users can now enable IMAP and sync their inbox across devices. Right now you'll only see IMAP settings in Gmail if you're using the English (US) interface, but IMAP will appear for all languages in the coming weeks. Learn more about IMAP at Help Center.
Gmail users with custom email addresses through Google Apps, can now enable IMAP too.
Video to learn how to quickly set it up on the iPhone.