While my original idea was to write a MT plugin to rotate the site's tag line, I decided to stick with the KISS principle, so I threw together some PHP code to do the job for me. I've stored the taglines in a text file, one per line, and a PHP function chooses one at random.
The functions that I use are defined in the file rand.php. I include this file in my main index template via the require_once statement, and then call the rand_line function to get my random tag line, which is stored in a variable. In my template, <$MTBlogDescription$> is replaced with a line like this one:
<?php echo $tag;?>
Right now there are only four tag lines to choose from, but I'll be adding more as I think of them. Feel free to e-mail me at rob@robruff.net if you think of any good ones that I could use.
I may yet write a MT plugin to accomplish this task, just for fun, if nothing else.