l've also had this problem with a user on entourage 2008 syncing with exchange
Firstly try resyncing your iphone with exchange, to do this
Open Settings - Mail, Contacts, Calendars - select your exchange account
once in the activesynce section turn calendars off and back on to re-sync
If that does not work have you imported calendar events from iCal into entourage? if so are they the events that are not syncing?
If they are you may need to recreate these events by removing and readding them in entourage
I