I have no problems as well. If I create a new appt and/or was invited to an appt it will appear in my Pre's calendar without any intervention. I got my Sprint Pre and on release day and have not seen any problems with my Pre's calendar picking up new appts. I do most of my Zimbra work either via Web interface on my PC or using Zimbra Desktop and it all comes down to my Pre without any issues.
Release 6.0.2_GA_1912.UBUNTU8_64 UBUNTU8_64 NETWORK edition + Mobile Option
Activesync with Moto Q9C, HTC Touch Pro, Palm Pro, & Palm Pre