I've recently got a 15" Macbook Pro ( knowing that a new model is most likely really close ) and I am happy with my purchase. Chances are that the new model will not be significantly better... of course, it will have a newer processor and maybe a slightly updated GPU ( although I saw no rumor / news regarding the GPU ), but in the end, it won't be as if the notebook doubles or triples it's performance.
I am a software engineer by trade and although speed is important to me, it does not justify waiting 1-2 months ( or more ) for a "slight improvement" ( I say "slight" because in "real life", 90% of the users will most likely not see a huge difference in speed - if any at all - when comparing Ivy Bridge to Sandy Bridge ). Of course, when people will start running benchmarks then the new model's score will most likely be higher and everyone will be "Woow, so coool!" but as I have already mentioned, most of them will surely not notice a real life difference anyway ( they can brag about having a higher score, but that's kinda it ) because they will never use the notebook to it's full potential ( based on your model choice, I am assuming you are not a professional editor who needs a crap-load of power for rendering / encoding video and such and nor will you be using the notebook to perform other high demanding tasks on the daily basis - I do not mention gaming because any model will run most of the games "decently" but since it's not a notebook designed with gaming in mind, it of course will not start blowing games out of the water - nor will the next model ).
Instead of waiting for the new model ( except if you are in no rush and you really want to see what the new model has to offer before you do anything ), I suggest you pull a tiny bit of money from somewhere and replace the HDD with an SSD. Having an SSD will lead to a visible speed improvement over the HDD ( something that even + or - 0.5 GHz of CPU speed don't always really show ) and I am 100% sure that you'll be extremely happy with the purchase, even if it's not the latest available model on the market.