Does a natural 20 give an automatic success against concealment?
I think not.
Quote:Concealment gives the subject of a successful attack a 20% chance that the attacker missed because of the concealment. If the attacker hits, the defender must make a miss chance percentile roll to avoid being struck. Multiple concealment conditions do not stack.
Quote:Attack Roll
An attack roll represents your attempt to strike your opponent on your turn in a round. When you make an attack roll, you roll a d20 and add your attack bonus. (Other modifiers may also apply to this roll.) If your result equals or beats the target’s Armor Class, you hit and deal damage.
Automatic Misses and Hits
A natural 1 (the d20 comes up 1) on an attack roll is always a miss. A natural 20 (the d20 comes up 20) is always a hit. A natural 20 is also a threat—a possible critical hit.
I think a natural 20 is an auto hit and then since the defender is hit, he can roll to cause a miss from concealment. If the attacker has blind fighting he can then roll that and if he gets through the concealment its a normal natural 20 (i.e. crit threat in standard d20).
I have no problem with a house rule that says a natural 20 that misses because of concealment becomes a normal hit but I don't think a natural 20 is an auto hit, a crit threat, and proof against concealment. That would mean you always have a 5% chance of hitting someone invisiable and when you hit, its a critical hit.