Straight from Capcom's official Japanese SFIV blog, it's a special message from SFIV's always-charming project manager, Shiozawa-san:
Its been so busy here lately, I'm beginning to become a bit absent-minded here.
Today, I am glad to introduce you to a Hidden Character in Street Fighter IV.
Actually, I wish I could formally announce this information, but I can't show that much yet so this will have to suffice… sorry!
Feast your eyes on this!
This is one Street Fighter IV's Secret, Unlock-able Characters, Sheng Long!"