Use post thumbnail as div background

<div<?php

    if ( $thumbnail_id = get_post_thumbnail_id() ) {
        if ( $image_src = wp_get_attachment_image_src( $thumbnail_id, 'normal-bg' ) )
            printf( ' style="background-image: url(%s);"', $image_src[0] );     
    }

?>>

Code should be pretty self explanatory – codex on wp_get_attachment_image_src.

As for how I’ve formatted it; I find this the cleanest, easiest to read, without outputting unnecessary whitespace:

<div style="background-image: url(http://example.com/wp-content/uploads/normal-bg.jpg);">