Jobs himself basically said as much during the introduction event. Something along the lines of, (paraphrasing) "Because of the millions of iPhone and iPod touch users out there, there will be millions of users who already know how to use the iPad."
To my interpretation, that is implying that due to their familiarity with the user interface, Apple is hoping that those millions of iPhone and iPod touch users will be significantly more likely to also purchase an iPad.
Yes, people who have iPhones AND iPod touches will be likely to buy one. People who own other Macs will as well. The person who said "the majority of iPad owners will be iPhone owners as well" was, as I recall, saying that they should be able to tether the two because they are already paying for 3G data. Fact is that it will likely be a small percentage of iPhone owners with an iPad, compared to all iPad owners - I'd hazard that iPod touch owners will be the biggest conversion group as there are more of them total and many of them might like to have an iPhone OS device with 3G but not want to switch phone service to AT&T.
And, what Jobs was implying with that statement, imo, was more an explanation of why they were using the iPhone OS - it works well, it's successful enough that millions of people have used it, so why wouldn't we expand on that instead of reinventing the touch OS wheel (by modifying OSX to have a touch interface).