From the day the iPad was introduced, Katy Huberty’s estimates of how many units Apple (AAPL) would sell — 6 million in calendar 2010 and 7 million in the first 12 months — were significantly higher than The Street’s. (See How many iPads will Apple sell?)

In a report to clients Wednesday, Morgan Stanley’s chief Apple watcher trotted out the results of a proprietary survey that suggest that iPad sales could even beat her numbers.

The survey of 2,500 American consumers found that 4.6% were “extremely interested” in buying an iPad and 16.4% were “somewhat interested.” (iPad skeptics will be happy to learn that nearly 80% were either “somewhat not interested” or “not interested at all.”)

Applying what Huberty calls a 77% discount rate on those forecasts (based on previous surveys of consumer interest in the iPhone and the iPod touch) and excluding non-Apple product owners (who are significantly less likely to buy an iPad), the survey suggests 4-5 million iPad sales in the U.S. over the first 12 months and 7-9 million worldwide.

Among the survey’s other findings:

• Evidence of strong “halo” effect. “20% or more of current Mac desktop, iPod Touch and iPhone owners are extremely interested in purchasing an [iPad] vs. 4.6% overall.”
• Trouble for the Kindle. “Owners of smartphones and eReaders are also more likely to buy an iPad.”
• It’s mostly a guy thing — for now. “Similar to our pre-launch iPhone survey in March ‘07, iPad extreme interest skews towards males but the current iPhone installed base is about half males and females.”
• It helps to be young and rich. “We see broad iPad demand across age groups but the highest demand from the 25-34 (29%) age group. Almost half (47%) of iPad users will have annual incomes $90K+, a number that is similar to the current iPhone installed base.” (See charts below.)

[Follow Philip Elmer-DeWitt on Twitter @philiped]