I updated the plugin today on 4 of my websites and I get the same problem on all of them.
This error message is showing in the recent comments widget and in the comments area in the backend. When I uncheck "Top Contributor Icon" the errors are gone again.
Warning: in_array() [function.in-array]: Wrong datatype for second argument in /home/piccaz/domains/piccaz.com/public_html/wp-content/plugins/top-contributors/functions.inc.php on line 555
Had to disable the plugin on my live sites for now, so here's a dev site where you can see the error in the right sidebar.
Thanks for this great plugin though, I've been using it for a long time and I'm sure this will be fixed somehow.