It's fair, provided that it's done... fairly.
1) If the PC can be removed from the story logically, I do so. (If it's a walking-around-town session, the PC can go off. If it's a dungeon, no.)
IF THAT FAILS...
2) Out of combat, the PC is run by me in a safety-first, coolness-last fashion. (Will not volunteer for dangerous stuff unless that's the PC's core function (like trap-disarming), will not participate hugely in social stuff if possible.)
3) In combat, the PC is played by the team as a whole. Everybody knows this. Because I, as DM, know the rules best, I often make choices for PCs that have unique skills other people don't recognize, but if it's at all risky, I run it by the group. ("Okay, none of you know how turning undead works. It could make this mummy run away, but it provokes an AoO, and I can't get the PC out of the way. Do you guys think that this is something Bob would have his PC do?")
4) I give a non-present player's PC a slight fudging edge in terms of survival, but if the dice clearly indicate death and there's no way to fudge it without a blatant rules violation or bad math, that's that. I'm more inclined to fudge in random fights, in which dying would be annoying and lame, than in big powerful fights where it's almost expected that somebody is gonna kick off.
5) I am only human. If the player misses the game because his girlfriend is in the hospital, I'm going to be a lot more inclined to reroll that saving through versus disintegration than if the player missed the game because he wanted to go out drinking that night and didn't bother to let any of us know until an hour after he was supposed to be there.
Not perfect, by any stretch, but that's roughly how I play it. I've had players come back to a dead PC and say, "Wow, damn. Wish I'd been there for that fight," and I've had players get extremely extremely angry because they felt unfairly targeted. In the latter case, the player was out drinking, and had missed the last month of sessions for various reasons ranging from "Was depressed" to "All my socks were wet". His character died by taking a full round of attacks from a green dragon, and he more or less saved the party by doing so. He was raised (I don't remember if it was Raise Dead or True Resurrection) at the party's expense, and he wasn't the first PC to die in the campaign. I could have had the dragon NOT target him, but if it's a choice between forcing someone who IS at the game to stop playing by killing THEIR character and taking a character whose player isn't there out of play, which one do YOU think I'm going to pick?
(I didn't go for the kill, but that was how the dice rolled. If he'd been dropped to -6 instead of -20, the dragon would have left him alone, and I'd have fudged the rules to say that he stabilized.)
But that's just me.