In a way, it's quite a lot of work. CheckGmail does not use the IMAP support of Gmail at all, instead relying on a stability in the HTTP handling from Gmail's part, requests all info via POST and GET.

In another way, it's not that much work, and will actually probably mostly be a question of ripping out a great deal of error-prone code (every small change from Google can be lethal in its current state - they are not committed to letting the HTTP API be stable) and replacing it with standards based IMAP requests.

When CheckGmail was created, I believe the IMAP alternative was not as developed on Gmail's side as it is today.

I also find the UI brilliant. Related is my longer comment on http://sourceforge.net/tracker/?func=detail&aid=3406322&group_id=137480&atid=738663