Hi there!
Just wanted to add my voice to the growing list of confused/frustrated.
Interesting thing is that I have literally just moved my mail hosting over to Google in the past week. Obviously, before moving from my previous hosting company, I did a fair bit of testing to ensure that Google could provide the solution I needed, namely IMAP access to my mail via 2 Macs and an iPhone. My testing showed this would work with Google.
So I changed over about a week ago. At first, everything was working brilliantly - my mails would appear instantly on my Macs and would come in to my iPhone on the scheduled Fetch every 15mins (Push not being supported on the iPhone for Gmail). Cool.
When I read a message, no matter where I read that message, both the Macs would update their unread marks, again almost instantly. The unread marks on my iPhone would update as expected on the scheduled Fetch cycle.
THEN something happened...! I don't know what has happened but only a few days into this perfect solution, suddenly my unread marks are no longer updating on the Macs. If I read a mail on a Mac then the unread marks sync fine with my iPhone on the scheduled Fetch cycle.
Aaaaarrrrrgggggghhhhh!
All I can say is feedback, feedback, feedback:
http://www.apple.com/feedback/mail.html
C.