I would bet there is no chance of seeing a Nahalem based MBP in 2009 or even Q1 2010.
The first Nahalem based mobile chip, Clarksfield, will be released Q3-Q4 of 2009, but the TDP is 45w/55w vs the 35/25w Penryns in the current unibody MBPs.
Battery life and heat seems to be somewhat of a priority to Apple, judging by their decision to include a hybrid graphics engine instead of a single 9600m gt or superior GPU. The technology for a 9800/8800 level GPU has been there and they had every chance to implement it, but obviously cost/heat/battery tolls prevented apple from utilizing it.
Considering both Penryn and Nahalem CPUs use 45nm technology, the quad core counterpart will produce significantly more heat. The higher TDP would also mean drastically lower battery life if apple were to decide to implement it in their MBP, not to mention will be significantly more expensive than the staying with the dual core.
MB/Ps may looks slick, but Apple can't perform miracles and are subject to physical limitations due to it's slimness - and that doesn't equate to good things in terms of internal hardware.
The second mobile Nahalem chip, Auburndale, will be released in Q1 2010 w/ a TDP of 45/35W - the most likely candidate if Apple ever decides to implement a Nahalem quad core mobile chip.
Apple seems to like the $2000/$2500 MBP pricing scheme - so until the manufacturing cost of parts or CPU/GPU goes down, you won't see a huge boost in upgrades.
If Apple were to implement a Nahalem chip, I speculate it wouldn't be until mid-late 2010 - and would most likely require a refresh of the design to accommodate the extra heat produced by the quad versus a dual core. Judging by the temps that 35w dual cores are pumping out in the unibody MBPs, there is no way, without a complete internal overhaul, that there will be adequate cooling for 4 cores in the current design.
Alot of people rag on Apple for not adding this.. or having that... but that's the price you pay for having such a sexy machine. People seem to like to think upgrading a processor is as easy as tossing it in and callling it a day. Apple has alot to consider and has to ride that fine line between sleekness and power so all their fanboys won't cry about it.
