Let's assume if pre-orders hadn't been available, everyone that ordered one would have shown up on day one to buy an iPad. If Munster's high-end numbers are correct, 700,000 iPads in one day is a phenomenal launch considering it's a brand new device in a brand new category. The iPhone 3G and 3GS sold one million units in three days and the first iPhone sold one million units in 74 days.
The iPhone wasn't in an entirely new category, however -- after all, many buyers had previously owned a mobile phone or smartphone before. Selling 700,000 units of a new device in a new consumer electronics category in one day is incredible.
It will be interesting to see if the iPad can break the one million mark by the end of its third day of availability on Tuesday (as many retail stores were closed for Easter this Sunday). With the 3G-enabled version of the iPad slated to launch later this month, there's likely to be another spike in sales for Apple's latest creation. Even though sales for the iPad will inevitably trail off after the launch day fervor, considering the strong sales numbers already reached, the iPad is definitely on track to beat the first iPhone's sales.
Thanks to Chris R. for his contributions to this post.