First of all let me just say that I absolutely abhor the "Limited Save Anywhere" concept used by several recent action/realism FPSs such as SoF2, IGI2 and Hitman 2. Instead of a quick, passive press of a button, the process of saving in these games becomes an annoyingly large thorn on the side of gameplay.
Gameplay is almost entirely centered around the "Number of Saves" and how best to use them with attainable threat information. Will it be a tough firefight around the next corner? Maybe I should save. How far away am I from my final objective? Maybe I should save. If I die here, how difficult would it be to repeat from the last save? Maybe I should save. I've been given a map of the entire level, at what points should I save? Instead of taking your focus away from saving and into more tactical decisions, this system actually emphasizes the unrealistically strategic relevance of saving.
Having said that, I don't think the "Unlimited Save Anywhere" concept works well for the linear, heavily scripted, horror FPS DOOM 3 is so far purported as being. Since the game is linear, the number of ways you can complete a mission are intentionally limited by the devs. So the excuse of trying different things just doesn't work as well as it would in a not-so-linear FPS. The memory of tension and drama a particular scripted scene is supposed to have invoked will too often be erased by seeing it just about a few dozen times in a row (For various possible reasons. Personally I'd repeatedly QuickLoad my way through such a scene with as little health lost.) The hand-crafted pacing of the game is thus lost akin to a badly edited movie. Lastly, what inspires horror and fear more than what you take in through your senses? The presence of brutal consequences of course. Something the Unlimited system was purposely built to tear down.
Ideally IMO, DOOM 3 would implement a flexible middleground such as the "Save at Predetermined Points" system used in Mafia and many console games or perhaps a combination of save systems such as the "Save at Predetermined Points" + "Limited Save Anywhere" systems used in Vietcong. Since little is known about DOOM 3's gameplay other than the aformentional generalizations, it would be arrogant of me to suggest a specific and detailed implementation.
Suffice to say I am pleased that Carmack is at least thinking about these things.
"Nothing livens up a robotic hymn of doom more than an amazing pair of jugs." - Brak