The general conclusion would be that the 40GB will become the top seller between the two models because of the cheaper price tag, but retail data from Amazon.com points that notion into a different direction. At Amazon.com, a large retail site which covers a wide range of products, presented data that shows the 80GB PlayStation 3 outselling the 40GB PlayStation 3. The actual numbers were not revealed, but it should still be noted that even at a higher price tag, people is still willing to pay the extra buck for a higher quality product. Not to say that the 40GB is inferior, but it does lack the ability to play PS1/PS2 games and features a smaller harddrive by forty gigabytes.
This should not be anything surprising because last winter, the overall demand for the 60GB PlayStation 3 was way beyond the demand for the 20GB PS3. Note that both of these models were discontinued in favor of the current ones.
As of today, the PlayStation 3 with all combined models has sold over 5 million units worldwide. That number is expected to jump tremendously in the next year.