Sync Your iPhone With More Than One Computer

Lets assume you want to sync with two computers, the one that you currently sync your iPhone with, we will call it the “regular sync computer” and the other computer you want to sync your iPhone with as well, we will call it the “new computer”.
On the “new sync computer” you want to sync with, backup “iTunes Music Library.xml” and “iTunes Music Library” (Tunes Music Library.itl on a PC) to a safe location as the hack involves modifying these files.

For full instructions on how to sync your iPhone with more than one computer head on over to iPhone Hacks.