Assuming you have control over the script you're using, it would be fairly easy to add a line of code that would, say, change an ALL UPPPER CASE post to all lower case. Mixed case would take a few more lines.

Assuming you've loaded the text field into $text, try

  # if ALL CAPS (no lower case), conver to lower case
  if ( 0 == $text =~ tr/a-z/a-z/ ) {
      $text =~ tr/A-Z/a-z/;
  }
[download]

I suspect, though, that you'll need to get more sophisticated, since users WHO WANT TO SHOUT can easily jinx this by adding 1 lower case character.