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Exactly which application would you like to run on the iPhone? iPad only application usually are setup for a different resolution than the iPhone and so other factors that the iphone won't be able to handel (since the application wasn't coded thinking about iphone support)
 
Some of the forum geniuses were able to get Netflix for iPad onto the iPhone, so I'm sure in some, but certainly not all, cases it is possible for the reasons mentioned above.

You can always look at the netflix thread to see how the .ipa file was extracted from the application as it sits in iTunes. Can't hurt to try!
 
Wired Mag

The ipad app thast I would like to see on my iphone 4 is wired magazine
 
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