Apple will likely add some new feature that is insignificant in the big picture, but the way they market it, you will feel like it is something you can't live without. That's how they sell computers. As a result if you get a MBP now, you will constantly be comparing it to the new one when it isn't really that much different than what you can get now. If you wait, there will be the possibility of something even better in a few more months and the cycle repeats.
To get the most enjoyment out of your purchase, you should buy a MBP now, or when you need one, then when the next one comes out, convince yourself you don't need X new feature. For example, Apple recently added an SD card slot,which my MBP doesn't have, but how often would I honestly use one? I use a USB SD card reader, and it works just fine. Another feature is the extra long life battery, but how often would I ever use my computer for more than an hour or two on battery with nowhere to plug in?
Possible changes in January are a slight CPU speed boost, but a 2.53 GHz is only 12% faster than 2.26GHz. For me, the newest 15" base model is only 5% faster in terms of CPU than what I have. They might increase the stock hard drive capacity or increase the RAM to 4gb, but those are cheap upgrades that you would probably do yourself anyway immediately after buying one.
The *only* feature I can think of that might be worth waiting for is Light Peak, but I don't think it will be ready for production in January, and even if Apple surprises everyone by adding Light Peak early and Intel uses their monopoly power to kill USB 3 (this situation is likely, IMO), it will be a while before lightpeak devices are widely available and inexpensive, and it will be a while before other brands of computers have lightpeak.
Apple might market the new feature(s) of the newest MBPs as exclusive (7 hour battery, magsafe charger), simplifying (SD Card slot, no button touchpad), fun (iSight), fast (CPU, RAM, or HD increase), or beautiful (unibody design, glass screen). Whatever it is, you need to make sure you don't fall for their marketing, because even if you don't go out and buy a newer computer, you will get less enjoyment out of the one you have if you are constantly comparing it to the newest ones.
Either get something now and enjoy it and don't stress about the newer models, or if you don't need it now, wait and hope you get lucky and then don't worry about the next next models. Remember, Apple wants you to want their latest computers and they are very good at it.