If my understanding of the rules is correct, at 0 HP you are still aware and can take certain actions, but taking a strenuous standard action such as attacking or casting a spell will drop you to -1 HP and render you unconscious. If you use that action to cast a healing spell on yourself, does that just drop you to -1 HP and waste the spell, or does it put you back into positive HP?
if you perform any standard action (or any other strenuous action) you take 1 point of damage after the completing the act.
The rules then go on to explicitly mention the possibility of healing thyself:
Unless your activity increased your hit points, you are now at −1 hit points, and you’re dying.
0 HP means that your character is conscious but crippled, thus they would be able to use an action to, say chug a potion. If you parse everything out the healing comes first in the chain unless it's a delayed effect but the standard action is complete and thus any resulting effects are in play and could bring you above 0 and without plinker damage. I also see nothing under the concentration skill that would demand a roll even when disabled since the damage isn't during the casting.
Below is from d20SRD:
Disabled (0 Hit Points)
When your current hit points drop to exactly 0, you’re disabled.
You can only take a single move or standard action each turn (but not both, nor can you take full-round actions). You can take move actions without further injuring yourself, but if you perform any standard action (or any other strenuous action) you take 1 point of damage after the completing the act. Unless your activity increased your hit points, you are now at -1 hit points, and you’re dying.
Healing that raises your hit points above 0 makes you fully functional again, just as if you’d never been reduced to 0 or fewer hit points.
You can also become disabled when recovering from dying. In this case, it’s a step toward recovery, and you can have fewer than 0 hit points
The damage comes only after the act, and even then, only if you are still Disabled. If an action brings you out of the Disabled state (which usually means that it somehow healed you back above 0HP), then at the time when you "should" take the damage, you are no longer Disabled, so the damage no longer applies.
In theory, if some effect were to cause you to stop being Disabled even though you are still at 0HP or less, then this should also prevent the point of damage. I can't think of any such effects off the top of my head, though.