Why am I getting and error from wp_get_attachment_image_src?

by

PHP does not support the function array dereferencing, which you want to do. In simple words – you can’t treat function as array.

What you should do is:

<div id="canvas" class="plan-page" style="background-image: url('<?php
    $image_src = echo wp_get_attachment_image_src( stripslashes($options['sitePlan']), 'full');
    if ( is_array($image_src) )
        echo $image_src[0];
?>');"></div>

Or to be even more correct:

<?php $image_src = echo wp_get_attachment_image_src( stripslashes($options['sitePlan']), 'full'); ?>
<div id="canvas" class="plan-page" <?php if ( is_array($image_src) ): ?>style="background-image: url('<?php echo $image_src[0]; ?>');"<?php endif; ?>></div>

PS. See an RFC on the subject http://wiki.php.net/rfc/functionarraydereferencing.