They only lowered the prices on the G1 iphone because the initial price was way too high, and they needed to gain marketshare more quickly. They raised the flash memory size from 8 to 16 GB to also gain marketshare. It was a brand new, revolutionary product that they needed to entice people to buy.
The iPad is a totally different story. The iphone platform is entrenched in the market now. People know what it is and why they want to buy it. There isn't as much of a need to prod people to buy it. And if 1M units already sold is any indication of the ipad's success, then don't expect Apple to change the formula any time soon.