Chris Roberts has revealed the death mechanic for his upcoming Star Citizen sandbox. In a lengthy missive on the game's official site, he outlines his plans for "a universe where time progresses, characters die, and new ones come to the front." He also talks at length about the the sense of accomplishment that's missing from games that let you save every five seconds. As such, the single-player portion of SC known as Squadron 42 will function much like Roberts' old Wing Commander and Privateer games in terms of death (i.e., you'll need to survive and complete missions, with no mid-mission saving, to advance).
The persistent version of Star Citizen calls for a different approach, though. Roberts acknowledges the immortal pod pilot lore and mechanics of EVE Online, calling CCP's method "clever" but ultimately saying that he's "not interested in making EVE 2.0 with cockpits."
He stops short of calling SC's mechanic permadeath, but he does note that every "death" will affect your avatar. "Eventually, after too many deaths, your character's body will just give out," he explains, "and instead of waking up in a med bay, you'll be attending the funeral of your fallen character from the eyes of the beneficiary you specified when originally creating your character."
There's much more to it, of course, including plenty of "death" examples and a FAQ, but you'll need to read Roberts' full blog entry for further details.
[Thanks to Eric for the tip!]