Syncing the iPhone with a second computer

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I'm really hoping someone might be able to suggest something to help me! My girlfriend has been using my MacBook for a while now and has been syncing her iPhone 3GS to her iTunes library on there and I have been syncing my iPhone 4 to my iMac. We will now be using my iMac as our main computer and have a combined iTunes library but I'm worried that when she syncs her iPhone for the first time she will lose everything. Is there anyway that she can start syncing with the iMac library without losing any of her settings/texts/contacts/apps etc?