There's a precedent:
* When the iPhone 3GS came out, Apple kept the 3G as an option to still buy, for a cheaper price.
* Same thing when the iPhone 4 arrived, you can still buy a 3GS for $100 less.
* And sometimes it's been done for the iPod touch--they kept one closer to the 2nd generation as the low end model when the 3rd came out.
I'm not sure how many people chose the lower end ones, especially for the iPhone. But it keeps real price pressure on competitors.
They could do the same for the iPad. When the iPad² comes out, presumably with cool new features like cameras, keep the current 16GB WiFi one as a lower end base model. And price that to kill competitors: $329, $349, $399, whatever. Most people will opt for the more expensive newer version--but it makes a really low price for competitors to beat (especially since they don't even really beat the current price without doing much smaller screens.)