I've had the Geo plugin working with my WP install for a little while but something happened and I believe it could be to do with the $postdata variable and a recent upgrade to 188.8.131.52. One of the functions in the plugin goes like this:
$geolocations = get_settings('geo_locations');
echo '<fieldset><legend><a href="http://www.asymptomatic.net/_wiki/GeoPlugin">' . __('Location', 'Geo') . '</a></legend>';
echo ("Postdata is: " . $postdata);
There's more to that function but it's crashing out on the !in_array line because $postdata doesn't seem to be an array. In fact it seems to be blank. Anyone have any idea on whether the recent bug fixes would have messed it up?