TNW suggests that the problem is the Flickr iPhone application, which will upload pictures straight to the service -- but not include metadata information like the fact that the pictures were taken with the iPhone. Still, even they sound a little skeptical: not all of that drop can be attributed to just the Flickr app's shortcomings. They also suggest that the iPhone is wearing out its welcome -- lots of people jumped to use it as a camera when the 3GS introduced a better lens and the video capability, and now in day-to-day use, they're not using it as much. Other commenters to the post suggest that the vastly improved Facebook app may be eating away at the use of the built-in Camera app.
I know that my photo habits for the phone haven't changed -- I use the iPhone quite a bit to take pictures, but looking at my own usage, I don't use Flickr nearly as much these days, as I use services like Twitpic and other sites built up directly around the iPhone. A drop in usage on Flickr doesn't mean people aren't using their iPhones to take snapshots -- they could just be sending their photos and media somewhere else.
Update: One of our commenters, echoed by our old friend & past colleague Barb Dybwad at Mashable, notes that Apple's change to the EXIF data recording for iPhone photos (breaking out the different iPhone models to indicate which specific phone took the picture) may be partly responsible for the drop.