If you need a new computer I'd get it now. Tomorrow would be a good time too because of the black Friday sales. Otherwise, it's probably a good idea to wait until after Macworld. It looks promising that there will be updates to the iMac at Macworld since the current versions have been out for about 7 months and the average time between updates is 7 months. So the iMac is coming due. But that is far from being a sure thing.
As far as getting lower prices, it's hard to say. Generally the prices stay the same and the hardware get upgraded, (like larger hard drives, new graphics cards and such). Maybe (and that's a big maybe) there will be a lower priced 20 inch if they use the new integrated nivida graphics chip they used in the new macbook.