If I turn PHP error reporting on in .htaccess, I see the error:
Fatal error: Call to undefined function has_post_thumbnail() in /wp-content/plugins/simple-nivo-slider/simple-nivo-slider.php on line 229
I am using WordPress 3.2.1 and have just installed the latest version of Simple Nivo Slider, which is v0.5.6.
After it is installed, I configure it to display 3 blog posts of category "Image" each containing an image with dimensions 960 x 304. None of theses posts have post titles - the title is blank, as I don't want any text to display.
The slider does not display at all. The source code shows it is being included okay.
The plugin page at WordPress.org says it is compatible with WP v3.2.1.