If you can hold off you purchase for a few months it may be worth the wait, especially if you are planning to get the 17" MBP.
The MBP line is due for an Upgrade going by previous trends, and the 17" is the one most likely to get an upgrades, as it was released in Jan 2009, meaning that its due for a refresh.
The proposed / rumored upgrades like core i5 / i7 are also more likely to be available on the 17" first as from what I understand they are more power intensive and run hotter, making them currently unsuitable for inclusion in more compact machines like the 13". There is probably going to be a graphics card refresh as well.