In Ubuntu Lucid, the Rhythmbox can detect your iPhone/iPod Touch by default and sync it with your computer. However, with the release of iOS 4.0, it breaks the sync and left many Linux users with system incompatibility.
Here is a quick and easy way to fix this:
Open a terminal and add the repository:
sudo add-apt-repository ppa:pmcenery/ppa
Next, type the following command:
sudo apt-get update && sudo apt-get install libimobiledevice1 sudo apt-get dist-upgrade
After the upgrade, you should be able to sync your iPhone/iPod Touch (with iOS 4) with Rhythmbox
Source: Ubuntu Forums
Image credit: karindalziel