I have just tried cradle syncing with our latest version (ZCO 5.0.15 -- not yet released), and a BlackBerry 8800, running BDM 4.6.0.12, and the calendar syncs fine. I don't think anything has changed in this area -- this has been working for me for a long time. I wouldn't think the type of phone makes a difference, but I suppose it's possible.
Some questions:
Has this ever worked for you?
How are you adding appts? Do you generally add in Outlook, Zimbra Web client, the device?
Do you have some log files you can send? Relevant ones would be:
zco logs
DesktopMgr.exe logs from the same directory as the zco logs
PtTrace.log (in Documents and Settings\<username>\Application Data\Research In Motion\BlackBerry\Intellisync)
RIM.log (in Documents and Settings\<username>\Application Data\Research In Motion\BlackBerry\Intellisync\<pin>)
WCESLog.log (in Documents and Settings\<username>\Local Settings\Temp)
Also, I'm not if you're also running Outlook when you do this. Make sure Outlook has shut down completely when you switch operations.
Thanks