Failing to create thumbnail, medium and large images
Right, the answer is in fact a little more subtle than I realised. Turns out that D7's imagecache replacement needs some attention in your nginx vhost. Without the 404s being sent to Drupal for the missing files, the images are not created - something I didn't realise initially, as I'd assumed they were created on upload.

location @rewrite {
    rewrite ^/(.*)$ /index.php?q=$1;
}

location ~ ^/sites/.*/files/styles/ {
    try_files $uri @rewrite;
}


It needs to be added to your server definition, and you should be ready to roll. Clearly, the path may need altering for your setup, but for mine it was.