Some of the games I played went with with the nat 1 and 20 for skill checks. Because sometimes you just f*k up no matter how good you are (even though it should happen a -lot- less than 1/20 of the time). However, we had nat 1 and nat 20 give a +/- 10 bonus to the roll instead of instant success or instant failure, in an attempt to have it make a bit more sense.
In my experience, and the experiences of others, the law of averages has no place at the table apparently, since those nat 20's pop up much rarer then 1:20, and those nat 1's seem to enjoy popping up all the time.
Anomander, your ruling here is something I actually do like. At my table, you can really only enjoy the fruits that a Nat 20 offer if you are making a check in a skill you're trained in. I mean, after all, if you're some expert in a skill, and some 9 year old kid rolls a nat 20 on an untrained skill check, well... as adorable as the precocious genius may be, the party may be excused if they drag the kid out back and stab him in the head off for "Knowing Too Much".
I may adopt the + / - 10 at my table and see how it runs.