Square Enix, the game's publisher, is moving the game to the next fiscal year, which begins in April 2011. The company explained the move in a written statement:
With weak sales performance of console game titles that have been newly released during the current fiscal year as well as harsh market feedback regarding a key title, the Group recognises the reinforcement of development capability in our Digital Entertainment segment as our most critical managerial issue.
We therefore have decided to spend additional time to further polish our upcoming game, DEUS EX: HUMAN REVOLUTION, resulting in a shift in release timing from our prior plan of the current fiscal year to next fiscal year.
Harsh market feedback? Sounds like Square Enix didn't want to risk having its feelings hurt once again. Delays stink, but better to have the game ready to go then have it be, you know, Final Fantasy XIV.
Deus Ex: Human Revolution is a prequel to 2000's Deus Ex, and right now, it's being polished.