I've said it once, I've said it a million times: Don't ever use the all blog for posts.

Make another blog, and then set it to be the default through index.php in the blog's options.

Then, if you want all the posts to be in the new blog instead of the all blog, open up conf/_admin.php in your favourite editor and look for the following chunk of code.

/**
 * Cross posting
 *
 * Possible values:
 *   - 0 if you want users to post to a single category only
 *   - 1 if you want to be able to cross-post among multiple categories
 *   - 2 if you want to be able to cross-post among multiple blogs/categories
 *   - 3 if you want to be able to change main cat among blogs (which will move the
 *       posts from one blog to another; use with caution)
 *
 * @global int $allow_cross_posting
 */
$allow_cross_posting = 1;

Change $allow_cross_posting to = 3.

After you've saved and uploaded the changed file, you will be able to change the main category from the all blog to the new blog. I recommend you change this back to what it was when you've finished.

Finally, go to the all blog's options, change it's access type to other blog through index.php and uncheck the boxes that change the blog list settings if you don't want it visible in your blog list.

This is what I've needed, thank you very much. Sorry--I haven't been around to hear you say this a million times:)