1. First, backup the "iTunes Music Library.xml" and "iTunes Music Library" (Tunes Music Library.itl for PC user). They are located in Music / iTunes Folder
2. On the machine your iPhone is currently synched with, open "iTunes Music Library.xml" with a text editor (TextEdit on mac, Notepad on Windows).
3. Find the entry between the
4. Now go to the computer you want to synch with and make sure iTunes is not running.
5. You will need to change the Persistent ID in two file. An XML File and a Binary File.
6. First, open the XML File "iTunes Music Library.xml" and find the entry between the
7. Save the XML File and close it.
8. Now open the binary file ("iTunes Music Library" on Mac and "Tunes Music Library.itl" on PC) in an Hex Editor (Windows : HxD , Mac : HexEdit)
9. With the command "Find and Replace" look for the ID you copy in step 6 and replace it with the ID in step 3. Make sure you search type is Hezadecimal not ASCII. Replace all occurences.
10. Save the file and close it.
11. That's it!
12. Launch iTunes on the second machine you wish to sync with and plug in your iPhone. When you select "Manually manage music and videos" you will no longer be prompted to delete music, nor will the files on your iPhone be grayed out.
Visit http://www.andrewgrant.org/2008/03/30/how-to-sync-an-iphone-with-two-or-more-computers.html for screenshot of the process