I know that some themes already do this (Panos made a really helpful list on his blog of all of them). But many don't including mine (Bueno) which I'm reluctant to switch from for other reasons.
It is such a great and simple feature to have the category definition "printed out" at the top of the category page!
Especially when the category might be one that readers are unclear about. My blog is educational/ tutorial-based and some of the categories are important for the content but not yet completely clear to beginners who are starting to learn the stuff. But I can't see why anyone who went to the trouble of defining their categories would mind having this feature--it's not necessary to enter a definition when setting up your categories (if the category itself is self-explanatory).
The blog I need help with is loopbraider.com.