Capcom is playing the paid upgrade game on Apple's iOS platforms as well as home consoles by releasing the superior Street Fighter IV Volt against the now-obsolete Street Fighter IV.  However, in an appreciated bit of fairness that is worth a mention, there are some secret unlockables waiting for iPhone/iPad owners who own both versions of the game.  Fire up the legacy Street Fighter IV with Volt also installed and the paid downloadable content — original Street Fighter II music and bonus costumes for Ryu and Chun-Li — will unlock for free.  Better yet, it's possible to easily unlock Akuma as a playable character in Volt provided that the original game is still on the iOS device.  Via GameFAQs, here's the secret code to make it happen:

1. Start Arcade mode, your cursor will be on Ryu. Tap to make costume select come up, then hit back.
2. Select Able. Tap for costume select. Hit back.
3. Select Guile. Tap for costume select. Hit back.
4. Select Cammy. Tap for costume select. Hit back.
5. Select Ryu. Tap for costume select.
6. Akuma should now pop up and be a permanent selectable character.

Normally Akuma can only be unlocked by completing the Arcade mode with all seventeen playable characters without losing a single match or playing one hundred multiplayer matches.  These methods take a lot of skill and/or time, so it's nice for Capcom to include a little shortcut for loyal fans who have already paid once to play Street Fighter on the iPhone and iPad.  With as much heat and scorn that we've all been throwing at Capcom lately for some greedy and unpopular decisions, it's nice to be able to spread a little praise around for a change.