How can I put an input element on the same line as its label?

It's possible without JavaScript, see: http://jsfiddle.net/Khmhk/

This works in IE7+ and all modern browsers.

HTML:

<label for="test">Label</label>
<span><input name="test" id="test" type="text" /></span>

CSS:

label {
    float: left
}
span {
    display: block;
    overflow: hidden;
    padding: 0 4px 0 6px
}
input {
    width: 100%
}

The reason why overflow: hidden is so magically useful in this instance is explained here.

display: table-cell is another option, see: http://jsfiddle.net/Khmhk/1/

This works in IE8+ and all modern browsers:

HTML:

<div class="container">
    <label for="test">Label</label>
    <span><input name="test" id="test" type="text" /></span>
</div>

CSS:

.container {
    display: table;
    width: 100%
}
label {
    display: table-cell;
    width: 1px;
    white-space: nowrap
}
span {
    display: table-cell;
    padding: 0 0 0 5px
}
input {
    width: 100%
}