On one of the WordPress-based websites I maintain, I have removed the category base so that, when browsing a category the url doesn't end with
/category/category-slug/. I accomplished that by putting a single '.' in the "Category base" text box of the Permalinks admin page. This worked fine until I upgraded the site to WordPress 3.0. The upgrade broke the category base such that browsing to
/category-slug/ resulted in a loop and eventual error. I was forced to back down to WordPress 2.9.2.
Fortunately, today's release of WordPress 3.0.1 has fixed the problem. I can now safely use '.' as my category base.