WebIS Knowledgebase

Each Outlook calendar event has an internal property known as "Message Class." When some Outlook plug-ins are installed (such as Live Search Maps), the setting for the Message Class property can change from its default of "IPM.Appointment" to something else. iPhone and iPod touch are only able sync calendar events with the Message Class set to "IPM.Appointment," therefore any calendar events created after installing certain Outlook plugins may no longer sync. IPM.Appointment.MP is used by an Outlook add-on named Cisco MeetingPlace.
 
http://www.google.com/support/forum/p/Calendar/thread?tid=057bcd71cbbe7476&hl=en
http://support.apple.com/kb/TS1465

To fix this:

1) open the registry editor (on x64, use the 32 bit one)
2) Navigate to HKEY_CURRENT_USER\Software\WebIS\WebISSync
3) Under this key, create a new Key named “Appointment”
4) Under the key you just created, add a new string value named “MsgClasses” with the value “IPM.Appointment.MP”
5) close regedit