I think it more comes down to whether or not there is any more possible good that could come out of you continuing to live than there would be in you dying. The burning building is a good example, but say you shot yourself in the head instead. Now barring some near-miraculous rescue that occured seconds after you pulled the trigger -- assuming you didn't wait until the flames were right there in front of you -- the only thing you really missed out on was a minute or so of incredible pain. In fact some good may even come of it; when the M.E. takes a look at your body he (or she if you're on Crossing Jordan or something) will be able to tell your family that you didn't suffer.
As for ending your life early because of a horrible fatal disease, I think that falls under the category of spiritual strength. Not everyone is strong enough to handle whatever life throws at them. I think as long as you did some deep inner contemplation, considered the impact of what you're considering doing -- not just on yourself but others as well -- and made the decision based on what would do the most good, then suicide wouldn't be a sin. (or at least not one that would get you sent to Hell or wherever) Of course there is always the possibility that you're supposed to stay alive a little longer to serve some hidden purpose, but that would also be something to consider and if applicable pray for guidance on.
In the case of committing suicide to save others that isn't really suicide so much as martyrdom. It doesn't matter if you do the act that causes your death, the situation which prompted you to do it wasn't probably your doing. Even if it was -- say you set a bomb to go off then changed your mind or found it would kill people, but disabling it would kill you -- I think that counts as a redeeming act, since regardless of whatever other sins you may have committed in the end you showed that you valued people and their well-being above all else.
Finally there's the case of the 15 year old committing suicide in a fit of teenage angst. (or something similar) A suicide motivated by depression. What occurs to me on this topic is that while the various religious texts say you need to ask forgiveness for sins you have committed -- i.e. suicide -- I don't seem to remember anywhere saying that forgiveness can't be asked for after you're dead. I'm not saying it's an open-ended "whenever you feel like it between now and eternity" kind of thing, but say the angsty 15 year old kills himself because some girl dumped him, and as his soul is floating around in the time between death and judgment he realizes exactly how poor of a decision that was and starts feeling pretty bad about it. Well according to the various religious texts if you repent for a sin and ask for forgiveness you will be absolved of it. I think the same goes for people who kill themselves because of a mental disease, assuming that when you're dead you no longer have any of your impairments in life. I'll bet you dollars to doughnuts that a schizophrenic who slashes his wrists because the voices told him to will definitely regret that decision.
Bottom line, it all comes down to whether or not it was a selfish act and whether or not you regret it if it was. The ones you'd really have to worry about were the people who planned their suicides for a while before doing it. If they spent so much time on it they might not consider what they did a bad thing, leaving a sin unforgiven.
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