I searched around and couldn't find anybody else who'd done this, so I guess I get to be the first one! (-:

In the _main.php file for your chosen skin, look for the line that says:

<?php $Blog->disp( 'tagline', 'htmlbody' ) ?>

and replace it with:

<?php 
global $Taglinecount;
$Taglinelist=file('http://www.yoursite.com/your_taglines');
$Taglinecount=0;
foreach($Taglinelist as $Tagline)
{ $Taglinecount++; }
echo $Taglinelist[rand(0,($Taglinecount-1))];
?>

replace "www.yoursite.com/your_taglines" with the file on your site containing taglines. Taglines should be one-liners only. By the way, I've even included HTML tags in one of the taglines in my file, and they came through nicely, so one-liner javascripts, special effects, etc., are not out of the quetion, here.

Enjoy!

EDIT: Whoops, forgot. Here's the [url=http://penguinpetes.com/b2evo/index.php]beast in action[/url].