I have managed to get imagemagick working (tested using the command line example on their website)
It also appears that the php extension is correctly installed: I can see it listed in my phpinfo(); list.

However, when I try to run the following code:

    $im = new imagick( 'examples.jpg' );
    $im->thumbnailImage( 200, 0);
    $im->writeImage( 'a_thumbnail.jpg' );

The execution stops at the second line, because it cannot find the thumbnailImage method.

What I can think of is that some user doesn't have enough privileges to access/run something else, but I obviously cannot give user access to everything.
Another possibility is that the PHP extension I'm using is not the suitable one for the latest imagemagick distribution.