Cartoonist & MAD magazine contributor Tom Richmond was commissioned to produce said caricatures for the iPhone app in question, and unfortunately they've run afoul of clause 3.3.14 of the developer agreement, the 'Apple's reasonable judgment' rule regarding potentially objectionable content. Richmond is scratching his head trying to figure out what about his caricatures could possibly be considered reasonably offensive, compared to some of the other fine entertainment apps already gracing iPhones worldwide. Still, it's at least consistent with Apple's previous rejections of things that are funny.
On some level it's unsurprising that an app filled with congressional bobbleheads is finding it a bit of a slog getting through review; that's a lot of potential angry phone calls for Apple to take.
[That's Dennis Kucinich over there.]