Arik Hesseldahl of BusinessWeek said today (here) that a group of independent software developers led by Patrick Walton have created what "appears to be the first downloadable program created for the iPhone." (More details here.)
But actually I think the first genuinely useful downloadable iPhone application might have been this one:
The programmer, a friend of mine named Garth Minette, discovered on July 28 that he could encode an entire application, 85 Kbytes in size, into an iPhone bookmark. So it works even when the iPhone is not connected to the network-- the basic definition of a "downloadable program."
This discovery was first disclosed by a mutual friend of ours here:
I can't say for sure that Minette was the first to discover this technique, but I can't find any prior implementations. If you know of any, please add a comment here. Thanks!