In a word, not Quad core C2Ds for the moment.
They are hotter chips, and the only Core 2 Duo currently used is Merom, the mobile low heat version which will not be going to quad core soon.
there may be a possibility of it, however, for either a new segment of apple desktops or a future rev. of the iMac, though the higher heat may be causing them to hold off until a form factor change.
quad-core Xeons will make it into the mac pro in the next few months, probably, for 8 cores total.