I had a request on #wordpress to be able to use http://yourblog.com/blog/archives/cat_name/ as a category permalink instead of the default WordPress way of http://yourblog.com/blog/archives/category/cat_name/ and I think I might have come up with a way to modify your htaccess to do just that.
Set your permalink template as:
Set your cat archives prefix as:
and use this file as your htaccess (or copy the contents to your htaccess) after replacing every instance of “blog” with the foldername of your blog. If you do not have your blog in a folder (installed into the root of your website) then remove every instance of blog/ in the following file.