http://wordpress.org/support/topic/bulk-password-reset-error?replies=2
Mentions the same warning. I see that the fix was applied over a year ago, but I get the same error in WP3.2.
I applied a the @ suppressor to fix it,
line: 188
echo ( @in_array($key, $send_roles) ? ' selected="selected"' : '');?>>
This could be added to the next release.