regards,
jez

1. Because visitors won't cache a re-usable header image as they browse, page load times will increase each time they browse a post in a new category.
2. The "branding" value of a static header may be lost. I think the distinctiveness of the header is the main way I personally recognise/ recall a site as a returning visitor.

There may be a compromise. At this point the best one I can think of is this:

1. A smallish distinctive header/ site logo that appears on all pages.
2. Use the CSS "z-index" property and positioning to layer a transparent category-specific .png on top of (or overlapping) the static header image.

Would this not achieve the required balance?

I have not ever tried using the category.php method, but that sounds like it would work nicely as well :)

You're right that to create a "site within a site" I would want just the single post pages and category pages to have a unique look—as I have above. So presumably what ever alternative method worked to include (say) header-default.php or header-koru.php would also work for two versions of archive.php which lists posts within category X (not to be confused with archives.php which lists posts by month and subject!)

The Codex says:

You might want to have different information or looks to specific category listings of posts. WordPress automatically looks for the category.php template file to generate the list of category posts, and if it isn't found, then generates them using the index.php. Using the template hierarchy, if it finds the category.php template file, it then looks for a category template file that matches the category ID number being sought, for example, category-2.php. If that template file matches, it will use that template file to generate a page view of all the posts in category 2.

I don't have a category.php file—but maybe if I made one (i.e. category-15.php) and used include to load the correct header, footer, etc?

Now if you wanted, you could insert the loop start code above everything else (not anything that outputs.. just the start of the while loop).

Then you would be able to use in_category to determine what header to show.

The main downside to that method is that you would only be able to use it in single.php for the actual post page, since it will only have one pass in the loop. If you did this to an archives page, it would go through the loop several times, trying to load the header, etc.. each time since it is after the start of the loop, and that would not work well at all.

Now if you want to have the custom header for category pages as well, one method would be to grab the category name from the URL. :)

I will be thinking about this, just in case there is a better option, but that would get it working for the cat listing pages as well as the single posts pages, which is all it would really need to be implemented for anyway, since archives pages etc.. will have posts from different categories together.

OK... do you think this would work for index.php?:

<?php if ( in_category(15) ) { ?>
<?php include (TEMPLATEPATH . '/header-koru.php'); ?>
<?php } else { ?>
<?php include (TEMPLATEPATH . '/header-default.php'); ?>
<?php } ?>
<div class="content">
<div class="primary">
<?php include (TEMPLATEPATH . '/theloop.php'); ?>
</div>
<?php get_sidebar(); ?>
</div>
<?php get_footer(); ?>