It seems that Dave's plugin writing addiction has rubbed off on me a bit. I had an idea for a possible MT plugin, but it turns out there is already a plugin that will do what I want.
My idea was to have a plugin that assigns a random string to MTBlogDescription, so I could rotate through multiple taglines. For example, on one viewing, the tagline may be "The Greatest Web Site. Ever.", and next time it may be "You will never find a more wretched hive of scum and villainy".
However, it looks like the MTRandomLine plugin will do what I want, so there's no need for me to write my own.
Update: It appears that I was incorrect. MTRandomLine does not do exactly what I want. It only inserts a new random line when rebuilding the template, not everytime the web page is loaded. Guess I should have read the plugin doc more thoroughly before my last post. Doh. Back to the MT Plugin forums...