Correct. You need to upgrade to 10.5.8 to sync with your computer. This goes for the iPhone 4 and the iPad.
I have a PPC G5 running 10.4.10. Just bought the software for 10.5.6. Then I will free upgrade to 10.5.8.
If the software requirement was 10.6, I would not buy the iPhone 4 nor the iPad as I don't want to buy a new computer just to sync my iDevices.