We've got Exchange 2010, and recently updated to SP2. Our clients are mostly XP and Outlook 2007, but some are on Outlook 2010.
I went down the path of upgrading to SP2 because a Mac user, who was running Entourage/Office 2008 was having inbound delays of up to 6 hours. First I upgraded her to Office/Outlook 2011, and that didn't resolve the issues.
Then I saw a bug or 2 about problems like this that might be resolved by updating Exchange to the current SP and RU. I installed the updates, and now this sort of sync problem seems to be wandering around all the mail clients.
My temporary fix has been to disable cached mode. Cached mode is nice though, and I was wondering if anyone else had had a similar problem, and found a real resolution to the problem.