The easiest way to do this now is with a flexbox:
http://css-tricks.com/snippets/css/a-guide-to-flexbox/
The CSS is then simply:
#container {
display: flex;
justify-content: space-between;
}
demo: http://jsfiddle.net/QPrk3/
However, this is currently only supported by relatively recent browsers (http://caniuse.com/flexbox). Also, the spec for flexbox layout has changed a few times, so it's possible to cover more browsers by additionally including an older syntax: