Most of the time I've found having 1/3 of the deck as land has been all well and good. In fact, Big Blue's insitence that it can't possibly be enough and we're all newbie morons if we don't agree seems a bit silly to me.
Of course you should go up or down from 20 land depending on the style of the deck, and such style choices may or may not have anything to do with the average casting cost of the deck.
But I suppose and objective way to approach this would be to do it mathematically. If you are running 20 land in a 60 card deck, then you should get a land one out of every three cards. Assuming you go first and don't draw on your first turn then by the end of turn three you will have seen at least nine cards; seven in your opening hand and the two others that you drew. Typically three of these nine cards will be land, so therefore you shouldn't miss a land drop until your fourth turn. If you don't have three land by the end of turn three then you're the victim of bad luck, not bad deck design, (And perhaps Big Blue is just the chronic victim of bad luck). If this principle is taken in to account then it would seem that the magical casting cost is 3. If nothing in your deck costs more than 3 than 20 is the most land you should ever play. If you don't run any sort of mana speed, deck thinning, or card drawing, then you are usually gonna have at least 4 land by turn 6 and 5 by turn 9. If all of your four and five casting cost spells can wait that long, then you probably shouldn't worry about playing with more than 20 either.
The three things I mentioned earlier - mana speed, deck thinning, and card drawing - are useful tools to making sure you climb the mana curve at a reasonable speed. As these things can help out more than just the mana curve, I'd argue that if you're deck needs more than 20 land then you just haven't built an efficient deck. Please note that I did say "maybe." Of course there are plenty of strategies and styles where you want to run more than 20, but even in these decks' cases you almost always don't want to run more than 24 land.
I understand a healthy fear of being "mana screwed"; it sucks, but so does having 8 land on the table and no spell to cast.