For Mac users, the iPhone 3G and 3GS required OS 10.4 to sync. And while we're a little late on reporting it, we know it will affect some users all the same. iPhone 4 requires OS 10.5.
For many of us, it doesn't matter. We're on OS 10.6 by now. But there's no real logic as to why, if older OSs can run iTunes, they can't sync with new iPhones...especially when you look at how well Apple supports iPhones on the PC. You can sync an iPhone 4 to any PC running Windows 7, Windows Vista, or Windows XP (SP3).
In other words, Apple will reach back in time 9 years to make the iPhone compatible on PCs but only 3 to make it compatible with their own computers. So much for the advantages of a closed infrastructure!
Of course, whereas you can't justify the logic at a technical level, the decision makes great business sense. This way, Apple sells the iPhones to Windows users and the new Mac OSs to Mac-based iPhone users.