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'});
Note that the negation pseudo class is in the Selectors Level 3 Recommendation and works in recent versions of Firefox, Chrome and Safari (at least). Sample code below.
<html>
<head>
<title>Negation pseudo class</title>
<style type="text/css">
div {
border: 1px solid green;
height: 10px;
}
div:not(#foo) {
border: 1px solid red;
}
</style>
</head>
<body>
<div id="foo"></div>
<div id="bar"></div>
<div id="foobar"></div>
</body>
</html>
Mozilla supports negation pseudo-class:
:not(.classname) input {background: red;}
See also: http://developer.mozilla.org/en/Mozilla_CSS_support_chart