It turns out there's a code to unlock Shin Akuma in Ultra Street Fighter II. Revealed by Capcom at this year's SDCC, you can play as the character's demon form by entering the following series of button presses while on the character select screen.
- Choose Ryu, highlight colour 1, then press cancel.
- Choose Ken, highlight colour 9, then press cancel.
- Choose Sagat, highlight colour 8, then press cancel.
- Choose M. Bison, highlight colour 7, then press cancel.
- Highlight the random select, then press L and R simultaneously.
Unfortunately, he doesn't stay unlocked, so every time you want to play as Shin Akuma you'll have to input the code again.