I have been trying for a couple weeks now to sync holidays to the iPhone with MobileMe. Finally, during Christmas I’ve finally had time to sit down and actually find a method. As some of you may have found out, MobileMe will not sync “subscribed calendars”, which is what all holiday calendars are. When you subscribe to a calendar from Apple, such as a holiday calendar, all you are doing is reading an .icns read only file. One way around this was to use Busy Sync and sync a google calendar with iCal, however that software costs $25. I wasn’t about to spend $25 on something that should be able to be done for free. So I continued my search. Luckily, I stumbled across a trick on macosxhints.com. It is a very simple method too. All you do is highlight the subscribed calendar and export it somewhere. Next import the same file you just exported and delete the original subscribed calendar. Now the holiday calendar appears as a local calendar and not a subscribed calendar thereby becoming able to be synced through MobileMe to your iPhone. Thank you to macosxhints for making this information available. I now pass it on to you.
Source: Mac OS X hints