As you probably already know, libgpod has included support for Apple’s iOS 2.x hash for a while now. With their new devices, Apple changed the hash again, but for some reason the change only applies to new devices – old devices running iOS 4.x still work. However, if you have a new device (iPad, iPhone 4, or iPod touch 4G), music sync does not work.
If your device is not jailbroken, you’ll have to wait until the new hash is reverse engineered. However, if your device is jailbroken, you’re in luck. As it turns out, the old DBVersion trick once again works to convince those devices to use the previous hash method.
In a nutshell, log in to your device via SSH, edit /System/Library/Lockdown/Checkpoint.xml, find the DBVersion key, change its value from 5 to 4, and finally reboot your device. This has been successfully tested on an iPhone 4, but I assume it will work for the others too.
Caveats regarding the iOS 2.x hash still apply. Specifically, libgpod needs some information to generate the hash. It can gain this information from a prior sync with iTunes, though this probably won’t work unless you sync again after changing DBVersion, and this hasn’t been tested. Alternatively, you can use this page to generate a HashInfo file for your device and manually copy it; this should always work.