How to Sync iPhone to Two Computers Tutorial
Kind of old, last topics i see for this is 2008/9 for this. But it requires messing with numbers with a hex editor. Mine just requires moving two specific files.
This was done with the latest (non-beta) of iTunes on both computers. My computers are a MBP running lion and a desktop PC running W7.
DO NOT HAVE ITUNES RUNNING
1: create backup of music and apps on the new computer. Unless its all on your idevice. Then you can just transfer purchases after its all done.
2: Open iTunes folder on your old computer.
For Mac it should be in your music folder, easy way to find it is just to open the finder, HDD, download, documents, and select the music tab on the left.
PC, open my documents and go to my music, or if you have my music on your start button.
3: Copy or extract the XML and ilb files. Put it on your desktop and email it to yourself, put it on a USB, whatever is easiest for you.
4: Get on the new computer and download or extract those files onto the desktop or somewhere easy.
5: Open the iTunes file on your new computer and replace both files (I recommend doing both at once)
Now the annoying part to this is that all the music will be your old music from the previous computer, but wont actually be there and all of the music you had previously in there will be gone. If you have it set up where itunes copies the music file and you dont have a separate folder you music youll want to make a backup of that music. If you have transferred your purchases from your device to the new computer you can go through the song list and not delete them otherwise, Just delete all of it and highlight your music folder and drop it into iTunes again. It will prompt you that it will delete all of the music next time you plug your device in, but if you have it selected as manually manage music you'll be fine.
Then plug in your iPhone and you'll be good to go.
I have heard you can do this with only the XML file but haven't tested it yet. When I get home I'll clean this up a little bit and upload a video.