I just did this. I was syncing with itunes on a pc and decided to now sync with my mac.
On my mac i registered it in itunes. Then i copied all my media from my old itunes and pasted it into proper locations in the new itunes.
I don't sync contacts, calendars, email so that wasn't something i had to deal with.
I did disable automatic syncing prior to plugging in the phone.
I plugged in the phone and right clicked on it and transferred all my purchases over to the mac.
Then i synced it. This removes your app folders on your phone which is a big pain in the rear. I couldn't find a way to avoid this. Then i had to recreate all my app folders on my phone and reorganize all my apps. It didn't remove any of my apps which i thought it was going to do. Most of my app data remained on my phone except for some games. I don't know why them but they may have been the most recent added games to my phone if that matters.
What i would suggest is to take screen shots of all your iphone "pages" so that you have a guide to recreate your app organization.
You will also have to sync your music, movies, etc back to your device. So go into their respective tabs and check what you want synced.
If you have multiple ios devices it gets more of a pain when you sync each device as itunes wants to add all the apps to the next device you sync. So when you plug in the next device before you sync it go to the apps tab on itunes and get rid of all the apps that itunes wants to add to the device that you don't want on the device.
Hope this helps....