If you see a great rate for December to Mexico, I'd recommend you take it now. August is a little too close to wait and hope you'd get a better rate.
BUT when you're on your August sailing - get 2 $100 Future Cruise Bookings (you each need one) and use it for some other cruise down the line. If you know where you'd like to go in 2010, you could book it while on board ... and it can always be transferred to your TA who might get you even better rates.