The changelog of the 1.4 release says that categories now have a description field.
I can't see one ;-)

Click on a category to edit one or create a new one, then you see a quite obvious big field (labeled "content" though for "space reasons"...). And there is also a custom field!
Both are not implemented in the themes right now. You have to add the theme functions yourself if you wish to use them.

Ooops, Don't know why but I was completely focused on looking for a one-line textfield. Not a complete wysiwyg editor field.
Sorry and thanks of course !

We figured a full field would be useful if you use categories as kind of sections instead of a pure blog way.