PlatinumGames loves to include an homage. So, when the devs were tasked with popping secrets and Easter eggs into Metal Gear Rising: Revengeance, they decided to include a secret unlock by using the classic Konami Code.
For those of you who don't know what the Konami Code is (either due to a lack of years or because you reside beneath a boulder), here is how you utilize the cheat:
At the Metal Gear Rising: Revengeance title screen / "Press Start" page on either the Xbox 360 or (PS3), use the D-pad to enter: ↑,↑, ↓,↓,←,→,←,→, B (O), A (X). In words that's: up, up, down, down, left, right, left, right, B or (O), A or (X).
If entered in correctly, you will hear a chime and a comment identifying the unlock. As a reward, you will be able to play MGR: Revengeance on Very Hard and Revengeance difficulty immediately.
Pretty neat! Thank you Platinum, Kojima, and Konami!