If you purchased items from the iTunes Store and the only place you have them stored is on your iPod, you can transfer them from your iPod back to any computer that is authorized for your iTunes Store account using iTunes 7 or later. This feature adds an extra layer of protection against losing your valued purchases.
If you want to transfer iTunes Store purchases from an iPod to a computer, be sure to authorize the computer before connecting the iPod. To authorize, follow these steps:
1. Open iTunes.
2. From the Store menu, choose Authorize Computer.
3. In the dialog that appears, the Apple ID button is selected by default. If you have an AOL account, select AOL.
4. Enter your Apple ID or AOL screen name and password, then click the Authorize button.
If the iTunes Store purchases on your iPod were purchased with multiple accounts, you will need to authorize the computer for each one.
To transfer iTunes Store purchases from your iPod to a computer, follow these steps:
1. Connect your iPod to the computer. Since your iPod is synced with another iTunes library, the following dialog will appear:
2. Click the Transfer Purchases button.
iTunes will begin copying any purchases made with any account that the computer is authorized for.
If when you connect your iPod you are not prompted by the dialog above, it could be that the option "Do not ask me again" was previously selected. Fortunately, there are two more ways you can copy your purchases from your iPod.
After connecting your iPod to the computer and the iPod appears in iTunes, you can choose Transfer Purchases from iPod from the File menu.
Or you can right-click (Windows or Mac) or Control-click (Mac only) your iPod in the iTunes Source list, then choose Transfer Purchases from the shortcut menu that appears.
Please note that this feature only works for items that were purchased on the iTunes Store. Any items imported from audio CDs or acquired from other sources will not copy from your iPod to the iTunes library.
http://docs.info.apple.com/article.html?artnum=305465
