Actually, there is a way you can sync your device to more than one iTunes library in different computers. The trick is making the device believe that it is the same library.
Each library has an associated identifier. By modifying one of the libraries and putting the id of the other, you will accomplish that.
In order to do so, select one of the computers, the one that you are currently syncing with, and find out its identifier. Look for a file called
iTunes Music Library.xml. In a Mac, this file is in the
~/Music/iTunes directory (though I think it can vary depending on what version of iTunes it was created with). In a PC, I guess it must be somewhere under the My Documents folder.
Open it with a text editor and find out a line which reads like
<key>Library Persistent ID</key><string>XXXXXXXXXXXXXXXX</string>
XXXXXXXXXXXXXXXX will be the identifier for that library.
Now, on the second computer, find out the same file, and, after making a backup (just in case), and making sure iTunes is not running, copy the identifier to a new document, because we'll need it shortly, and then replace it with the identifier from the first computer. Now, we're halfway done. The following step is the difficult one.
With a hex editor (which is an editor that allows editing the bytes of a binary file; my PC using mates recommend Ultraedit, while I on the Mac like HexFiend), open a file called
iTunes Library in the same directory (remember, make a backup first), and do a search for the same string (or rather, byte string) that you copied and replace it with the new one from the first computer.
After performing these steps, you should be able to sync both computers with the same device without problems.