If there's any other poor sod out there looking for the answer to this one (I was...)
Here's the proper way to do it:
<?
$im = new Imagick();
$im->setBackgroundColor(new ImagickPixel('transparent'));
$svg = file_get_contents("path/to/test.svg");
$im->readImageBlob($svg);
$im->setImageFormat("png32");
header('Content-type: image/png');
echo $im;
?>
Note the "setBackgroundColor" first, the order matters.
Also note the image format is "png32".
Hope this helps someone...