Steveroo, no -- 1's are only autofails on attacks and saves, not skill checks. And 20's are only autosuccesses on attacks and saves -- not skill checks.
OTOH, IMC I use the "roll a 1, and it's a -10 on the die; roll a 20, and it's a +30 on the die". So to autosucceed on a defensive casting, you would have to have a Concentration Modifier equal to 25 + spell level.
Alternately, you could apply the Open-Ended Rolls optional rule for the ELH -- if you roll a 1, you subtract 20 from your running total and roll again; if you roll 20, you ADD 20 to yoru runnign total and roll again.
So, even with a +100 Concentration modifier, defensively casting a 1st level spell (DC 16), if you roll 1 five times in a row, and then roll 15 or less -- you fail. Vanishingly unlikely, but still possible to fail. OTOH, with a +5 concentration modifier, and trying to defensively cast a 9th level spell (DC 24) while standing next to a 40th level fighter with the Spellcasting Harrier feat (for a -20 modifier to your check), if you roll a natural 20 once, then a 19 or 20, you still have a (slim) chance to succeed.