Helpful guide is from: Shiny Things: How to sync an iPhone with two (or more) Computers
Backup Your Library
On the machine you want to sync with, backup "iTunes Music Library.xml" and "iTunes Music Library" (Tunes Music Library.itl on PC) to a safe location. These are the files you will be modifying.
1) Find your iTunes Library ID
On the machine your iPhone currently syncs with, open "iTunes Music Library.xml" with a text editor and find the entry between the
2) Change your Library ID
iTunes stores your library information in two places. An XML file and a binary file. You now need to change the ID in both so they match your other library.
Make sure iTunes is not running.
On the machine you want to sync with open "iTunes Music Library.xml" in a text editor. Again, find the entry between the
When you have a copy, replace this entry with the one you copied in part 1. Make sure you do not change anything else, and the length of the entry is 16 characters/digits. Save this file and close it.
In the Hex Editor, open "iTunes Music Library" (Tunes Music Library.itl on PC).
Select "Find and Replace" from the Edit menu. Make sure "Hex" matching is selected (not ASCII). In "Find" enter the ID you took a note of in Part 1. In Replace, enter the ID you copied at the start of Part 2. Choose Replace All, there should be one match.
Save this file and close it.
Start iTunes on the 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 erase your data, nor will the files on your iPhone be grayed out.