Do you have the .MobilMe service from Apple?

If you do, then the syncing of iCal & Address Book are done through it. Make sure that you have MobilMe sync turned on and that it is set to Sync Automatically.

If you do not have that service, attach your iPhone, select it in iTunes and check ALL the tabs to check their settings. You may find that updating iCal and Address Book settings have been changed.

Good Luck