The next time prices come down will mean the new one is coming out (most likely) - the newest version of the kindle has been kicking about for a while now.
I'm quite tempted by one, but I may just be addicted to things with screens.
As Rojash points out: the market cycle means there is never an optimum time to buy.
Would you be happy with the current model? If so either buy it now, or wait for the price to come down. If you think it is lacking in some way, wait for the newer one and pay a premium on it.
All the best,
Spare Ed