there's no reason not to switch to Core 2 Duo as they sell for the same price as Core Duo and are drop in replacement.. the only possible reason for the delay is supply shortage.. it's not unusual to see PC vendors announce new processors even though they are unable to ship them at that time, but at the same time they keep options for the older model, which ships instantly.. Apple thinks differently, they will announce the update when they're ready to ship and starting that day they cease selling the older models.. I think thats fair.. the only difference is that they don't make announcements several weeks in advance..
Apple has no reason not to follow Intel's speed bumps, because on each speed bump the prices shifts towards the lower models, in other words Intel keeps the prices the same but speed bumps at every level.. if Apple does not update, they're keeping extra money in their pockets..
as for the updates, I'm curious wheter it's gonna be just a processor switch or wheter they will also move from ATi to nVidia as they did in iMacs.. ATi is now owned by AMD and maybe somehow Apple bets more on nVidia because of using Intel processors only.. also I don't expect MXM slots for video cards for the same reason there's no processor socket in MBP, they ough to keep this as tight as possible..