Not CSS selectors

With current browser CSS support, you can't.

Newer browsers now support it- see Sam's answer for more info.

(See other answers for the alternatives in CSS.)

If doing it in JavaScript/jQuery is acceptable, you can do:

$j(':not(.classname)>input').css({background:'red'});

Mozilla supports negation pseudo-class:

:not(.classname) input {background: red;}

See also: http://developer.mozilla.org/en/Mozilla_CSS_support_chart

Wouldn't you do that by setting the 'global' background to red, then using the classname to alter the others?

input { background: red; }
.classname input { background: white; }

I would do this

input { /* styles outside of .classname */ }
.classname input { /* styles inside of .classname, overriding above */ }

There is no way to select the parent of matched elements with CSS. You would have to use JavaScript to select them.

From your question I assume you have markup that looks more or less like this:

<form class="formclassname">
    <div class="classname">
        <input />  <!-- Your rule matches this -->
        <input />  <!-- Your rule matches this -->
    </div>
    <input />  <!-- You want to select this? -->
    <input />  <!-- You want to select this? -->
</form>

One option is to add a class to a higher element, say the <form>, and write a rule to style all of the inputs of the form. I.E:

.formclassname input {
  /* Some properties here... */
}

Or

.formclassname > input {
  /* Some properties here... */
}

If you want to select them based on the fact that they are not inside of an element with a specific class, you're out of luck without the use of JavaScript.

I think the closest you can get is to only affect direct descendants with a declaration

This code for example will only affect input fields directly under divs with class "maincontent"

div.maincontent > input {
  // do something
}

Inputs are a bit annoying because, unlike most other html elements, there isn't necessarily a way of resetting all the css properties back to their default value.

If the styling is non-critical (ie a nice to have but doesn't affect functionality) I would use jQuery to get an array of all the inputs, check their parents, and then only carry out the styling on those outside that div. Something like:

$('input').each(function() {
     if($(this).closest('.classname') == false)
     {
           // apply css styles
     }
});

(By the way, I'm no jQuery expert, so there might be some errors in the above, but in principle something like this should work)