Thanks for the reply. What I mean is that the header in my main blog which contains a sitewide search, a javascript menu (with a sidebar, where I've placed sitewide tags, categories, posts etc) and I would like that header to appear across all sites - and that it uses the same css as from the main blog, and calls back to the main blog for the results for the sitewide search.
And no, all the blogs are not using the same theme - and I don't mean that this sitewide header would REPLACE the themes header, just sit on top of it. Just like the admin bar, but with all the functions (and theme) like in my main header blog.
Maybe it's impossible. I thought that you might be able to insert this on top through an iframe or some wrapper script, but I'm quite the novice when it comes to php programming.
I would LOVE to hear if anyone had any suggestions on how to accompish it.
Thanks so much!