In the past, when Apple hasn't been able to immediately release items was when they were introduced prematurely at MacWorld. I think a Mac notebook update will not happen at WWDC but the week before or shortly after. The focus of WWDC is software and iPhone OS. If Apple updates Mac notebooks at WWDC, I would be surprised. If Apple ever updates Mac notebooks again without having them immediately available, I would be surprised. Remember at MacWorld, neither the iLife nor 17" MBP were ready, and Apple was forced to introduced not on its own timeline. I think with Apple getting out of trade shows, and focusing on its stores for sales, it will introduce new Macs whenever they are ready. The iPhone could be introduced at WWDC with a wait, because the iPhone has OS software that developers can develop apps for, so a real reason to introduce before ready.
My two cents.