Most of the people who lined up in New York and Minneapolis to purchase the iPad on Saturday were already committed Apple (AAPL) users, according to the results of a survey of 448 iPad buyers issued early Monday by Piper Jaffray’s Gene Munster.

The full results are pasted below the fold. The key findings:

• 74% were Mac users (26% owned another kind of PC). 96% planned to continue using their computers.
• 66% owned iPhones. Only four or five respondents (1%) thought they could replace their iPhone with an iPad.
• 13% owned Amazon (AMZN) Kindles and 58% of those planned to replace it with the iPad.

• The $499 16GB iPad was the most popular (39%) followed by the 32 GB (32%) and 64GB (28%). When the iPhone first launched, only 5% bought the low-end 4GB model.
• 74% planned to use their iPads to surf the Web; 38% to read books; 32% to e-mail; 26% to watch video; 18% to play games and other apps; 8% to listen to music.
• 78% said they didn’t consider any other gadget before buying an iPad. 10% were thinking about buying a Kindle, 6% a netbook, 4% a laptop, 1% an iPhone, 1% an iPod touch.

“We believe that Apple has successfully carved out a new category of mobile devices between the smartphone and the laptop,” Munster concludes, “apparently without cannibalizing its own iPhone and Mac sales.”

Based on the size of the crowds he observed, Munster has more than doubled his estimate of iPad sales for calendar 2010, from 2.7 million to 5.6 million.

Below: The survey results, with comparisons to surveys conducted at each of the three iPhone launches.

See also:

• The iPad and the failings of the computer industry
• New iPad launch-day estimate: 600,000-700,000 sold
• Inside the iPad launch day hoopla
• Dawn of the iPad
• Who camps out for an iPad?

[Follow Philip Elmer-DeWitt on Twitter @philiped]