They're not going to change the hardware. You might be thinking about the PS3 Slim, the DSi, the Gameboy Advance - but even if you are, those were many, many years into their cycle.
As has been said, they would never redo a product with different hardware unless there was a catastrophic issue.
The only time I've really seen a development change was in the Xbox 360 units, they made minor changes to the chipset, but because of how compacted the iPad is, I cannot imagine them swapping anything out.
Simply put - either buy now or wait a year and a bit to get the upgraded version.