I think I can simplify this topic even further, for those who are still in need of help...it only requires ONE purchase from the Apple Store online.
If you just bought the new iPhone 5 and have discovered that you can not sync your iphone to your MacBook which has Mac OS x 10.5.8 software, because it is not compatible with iTunes 10.7 or higher.
1) Go to the Apple Store online (or contact your local Apple retail store to see if they have stock)
2) Purchse Mac OSX Snow Leopard v.10.6.3 (Cost $19.99...ships out 24 hours/arrives in approx 2-3 days)
3) Back up your MacBook to an external drive (do not skip this step)
4) Insert the Mac OSX v.10.6.3 disc (takes about 1.5 hours to download/install)
5) Upgrade your iTunes software from the iTunes Store (currently the latest version is v.11.02)
6) Back up your old iPhone, then connect & sync your new iPhone 5
7) Finished...just did this today and I have not incurred any drain on my system speed at all! 🙂
Good Luck everyone!