Put iPhone into DFU mode.
Plug iPhone and a iPod into the Mac.
See iPhone in iTunes in recovery mode.
I found if I used an iPod that I could get iTunes 8 on a Leopard 10.5.6 MacBook to pickup my iPhone 3G in DFU, iTunes 7 & 10.5.5 had no issues, updated version dose.
Also i have a iTunes 7.7 app along side the iTunes 8 as I have read that iTunes 8 will refuse to update using a customized firmware, I don't need to jailbreak my phone so never tested updating the firmware on my iPhone, i have only found a way to get OS X & iTunes 8 to reconize the iPhone in DFU mode.