According to Sonic Team, the iPhone version of Sonic 4 is the reason why the game has taken so long to be released on all formats.
Speaking to CVG, Takashi Iizuka said "Originally the game was supposed to come out in summer this year. While it wasn't really apparent at the time, we announced this as a PSN, XBLA and WiiWare title but at the same time the iPhone version of Sonic 4 was also in development."
In an interesting turn of events, it now seems as though the console versions have been based on the iPhone version, not the other way around.
"What they did was they kept the iPhone version closer to the original and they went back to the console version and changed some stages to be better playable on console. That's the reason behind the delay and the extension in the development schedule."
An interesting explanation - hopefully the game still feels like a console native title.