I'd like to clean my slate -- entries and categories-wise -- and start with freshly imported blog entries, but keep all my other MT settings and templates. Which tables should I truncate to do this without damaging the rest of my setup?

I've never done this specifically, but what I would do would be to follow this http://www.elise.com/mt/archives/000255mov...or_web_host.php
and back up all of your information and then start with a nice fresh installation.

The other thing you can do is log into MT, click Edit Entires, and then "open power editing mode" which will let you select and delete entries and attached commnets on a large scale with ease.

In my own personal opinion MT databases are very touchy things and I wouldn't try and mess with them, though someone else might tell you differently.