If they made the macbook pro quad core right now, it would literally set your desk afire.
The whole machine could just spontaneously combust once you load it up to 100% on all 4 cores.... Heck, even 50% might do it XD
The question isn't when Apple will do it, it's when the technology will be available. There is no mobile quad core cpu right now. It simply doesn't exist. Last I heard, they're due in 2009. After that, Apple (along with everyone else) will offer them.