A trusted source has told Joystiq that "Harmony of Despair," the title of a recent European trademark, is indeed a Castlevania game. Our source indicates that the game will be a 2D multiplayer Xbox Live Arcade title, featuring six-player co-op and a versus, fighter-style survival mode.
These details follow a recently-pulled post on never know tech, which purported to showcase screenshots of Harmony of Despair. The site described the game as a "Metroidvania" side-scroller, featuring characters from recent Castlevania games. The characters, however, appeared to be rendered as tiny GBA/DS-esque sprites, and the game camera is apparently capable of being zoomed out to reveal entire levels (presumably in place of a map screen; or to show multiple characters in different rooms).
So it looks like we might be getting the long-awaited 2D "high-definition" Castlevania, after all. Although, "just make the GBA sprites really small on the screen" isn't exactly the technique we were hoping for.