Some possibilities:
1) There's a strong possibility that the 17" MBP is going to be discontinued. Rumors abound to this effect.
2) Apple's history reveals that in the event they _are_ keeping it in the lineup, it will still arrive months later than the 15". That's the way it's been each time the MBP line has been revamped.
The current model is excellent, I know I have one. It's the fourth new 17" I've had and I'm completely satisfied. Fully debugged and refined there's no reason not to keep one of these.
The mentality of having to have the latest, is not always wise or beneficial. Newer is not always better. Here's an example. I bought one of the newly redesigned 2010 MBA's. It was OK, but the 2011 was so much better, that I sold the 2010 and bought a 2011.
In addition, there's a great likelihood that if Apple does decide to keep the 17" size, the new one may have several bugs and annoyances. If you don't mind going through that, then gamble.