Another quick and easy technique which eludes some of us! While the use of rgba for background colors is not supported by all browsers, it can still be used with a fallback:

div {
/* Fallback to solid white */
background: rgb(255, 255, 255);
/* White with 50% opacity */
background: rgba(255, 255, 255, 0.50);
}
div p {
/* Text inside the div will render at 100% opacity */
color: #000;
}

Just remember that the fallback should be declared first since the last instance of a property (that the browser understands) takes precedence.