How to sync a Drupal calendar with an iphone/ipad Calendar?
The Date iCal module can be used for syncing a Drupal calendar in both directions. Some more details about it (from its project page):

... your one-stop shop for iCal support in Drupal. It provides a plugin for Views to enable exporting your site's calendar as an iCal feed, and a plugin for Feeds to enable importing external iCal feeds into your site's calendar.
Any entity which contains a Date field can be utilized by Date iCal for import and export of iCal feeds.

So it provides:

a plugin for the Views module to enable exporting a Drupal calendar as an iCal feed.
a plugin for the Feeds module to enable importing external iCal feeds into a Drupal calendar.