Nope, it's not "standard" at all... It's more like "just enough to get by"
The guy who
invented IMAP said that if he were to rate the implementation it would be "quite damning." (see
http://www.wired.com/software/webservices/news/2007/10/imap )
Also see
http://weblog.timaltman.com/archive/2008/02/24/gmails-buggy-imap-implementation for more information on some of the specific deficiencies.
To be fair, GMail IMAP has gotten better than it was when they first rolled it out (my early experiences were that sometimes it wouldn't even download complete messages), but the way they are doing things is
definitely not in full compliance with the IMAP RFC, as they've had to take several shortcuts to try to make it "fit" with the way that GMail otherwise works.
As others in this thread have mentioned, I've been using Apple's Mail.app with a half-dozen other IMAP accounts for four years now, including MobileMe, GroupWise, MS Exchange, and Cyrus IMAP servers, and I've never had the kind of weird little idiosyncratic problems I've had with GMail's IMAP implementation.
I'm not suggesting it's
entirely GMail's fault, mind you.... Other IMAP clients seem to be able to deal with it more successfully, but at the same time there's something between the
two that is causing problems. Also, bear in mind that many other software developers frequently
deliberately program around buggy IMAP implementations to provide the widest possible range of support (I was a beta tester for a Palm IMAP app a few years ago, and we went through about a dozen beta releases before we got it working right with GroupWise's IMAP, which was abysmal back then).