Yes, I have the basic one with the solar panel that flips up - I would imagine it would be easy to fashion some sort of cover for the iPhone (towel/bandana/cardboard) because it would be directly facing the sun while it charges. I actually use an mp3 player with it, so I use the connection on the back to input music (and put a towel over the mp3 player). I like it because it's compact, completely rechargable as long as there's sunlight and it sounds great.
I don't know about retro-fitting it, because that would definitely interfere with the design, but it is probably possible.
"I'd rather die while I'm living than live while I'm dead." - Jimmy Buffett