At the embarrassment of entering this discussion a month late, I would like to add my 0.02.
I use a house rule where everyone (PCs and NPCs) die at -(CON score) and are disabled at 0 to -(CON modifier). This means your average commoner with a CON of 10 still dies at -10 and is disabled at 0 hp as in the standard rules (characters with a CON of less than 10 are still disabled at 0, though they die at whatever their -(CON score) is). But it allows PCs (and NPCs) with above average CON scores to have a few rounds of action before they fall unconscious. Any strenuous actions still drop their hps by 1 (they're stable if they don't do anything strenuous), but unless they drop below their -(CON modifier), they remain conscious.
So for example, Bob the fighter has a CON of 16 (a +3 modifier). He is disabled (conscious) from 0 to -3, dying (unconscious) from -4 to -15, and dead at -16.
You can easily modify the Diehard feat to allow characters to simply remain conscious during the dying phase, as determined by their CON score.
To a certain extent, I like the idea of factoring levels into a character's negative hitpoint pool but I'm also leery of having a massive negative hitpoint pool (more than say, -20)-- in my mind, a higher-level character already has more hitpoints to represent the fact that he's harder to kill. If a fighter with 150 hitpoints has been reduced down to less than a quarter of his hitpoints, he should start looking to withdraw or fight more defensively, especially if he knows that he's fighting a creature that can hit hard, instead of relying on having a lot of negative hit points to absorb that critical. (Then again, when do players withdraw from combat unless they're dropped?

) I mean, if you're a high-level character and you're so badly hurt that a decent sword swing can take you out, shouldn't you be playing it safer (unless you're aiming for a glorious, gory death)?
In the end, I guess it depends on what your priorities are. I'll admit my system tends to keep lower-level characters alive (especially mages, who might be 3rd or 4th level before their positive hitpoints exceed their negative... weird, but doesn't bother me too much) and is of a more minor benefit to higher-level characters. I will add in one caveat: I do allow temporary ability score modifiers to affect the character's negative hit points. That is, if Bob the fighter normally has a CON of 16, but is wearing an amulet of health +4 (or has bear's endurance cast on him) so that his current CON score is 20, he dies at -20, not at -16. Likewise, if he's been CON-drained to 12, he'll die at -12, not -16. That said, higher-level characters do have ready access to spells and items that can boost their CON scores, and therefore, their negative hitpoint score as well.