I would say wait it out. People that say only the high end MBPs will be updated are only thinking about the 6,1 and 6,2 kext files. Apple has never updated a high-end MBP and not the low-end of the same size display MBP. Apple has updated 15.4" and 17" MBPs at different times. I definitely believe ALL MBPs will be updated at the same time, as the 17" unibody introduction was introduced later because of a delay for larger unibody cases.
I believe waiting for certain processors is one of a few major reasons to wait. My reasons to wait would be as follows:
1. Form Factor (currently Unibody and will be for years) - No reason to wait here.
2. CPU 45nm process (currently Penryn Core 2 Duo) - Real reason to wait here, as 32nm Arrandale CPUs are to be released on January 5, 2010. Everyone thought for certain this would be the CPU used in the next MBPs. However, since the iMac got a different CPU people are thinking high end MBPs will be different... I don't see that happening.
3. Graphics - Real reason to wait. Chances are we will see a MUCH better graphics selection in the next MBPs. Probably switch to far less problematic ATI card.
4. MAJOR tech update - We are currently waiting on USB 3, BluRay, OLED displays, faster SATA, and some would say light peek although most just say that due to the rumor here. The one worth waiting on is USB 3, but Intel is supposedly putting that off until 2011. So, forget waiting that long.
I think Arrandale is a worthy reason to wait and also an upgrade of the graphics card.
If you wanted any other Mac, I would say go ahead and buy it... but the MBP has a few big possibilities in its next upgrade which could happen anytime between Christmas and March. Probably closer to January 1st than anything.
Good luck.