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well, back when you had the original iphone, nobody knew the following:

1. Their was no app store, so people didnt know about apps.
2. most people didnt think it could play their music as well..at least a few friends that saw my original iphone didnt know that.
3. The price was outrageous.

So factor in all the current iphone users, and how everyone knows about the app store now, and of course more people are interested.
 
The original iPhone was $499 for a 4GB model without 3G and no App Store. Compare the interest between the iPad and the iPhone 3G, which started at $199.
 
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