Well, that's odd - the forums seem to have marked my post as closed before it's even been opened.
So, reposting my simple bug report:
On line 96 of yahoo!-wheather.php, if credits are not shown, $credit is undefined, throwing a "Notice: Undefined variable" when WP_DEBUG is set to true.
The simple fix is to define it as empty on the previous line, ie:
$credit = ''; if ( $show_credit ) $credit = ' & <a href="http://zourbuth.com/plugins/yahoo-weather/">zourbuth.com</a>';