As DoingItWell explained - CSS allows you to position elements on a page in pretty much any location you like, irrespective of it's position in the page source.
The first call to google in your page source code will pull the highest paying ads, but that doesn't mean they have to appear at the top of the page...
On one page I have 3 Ad blocks - a banner (low CTR of about 1%) a center 250x250 block (pretty good at 4%) and a skyescraper (about 1.5%).
I pull the center block first in the page source (highest paying ads to the highest CTR block), skyescraper second, and banner (even though it appears on screen at the top of the page) last.
Hope that makes sense - a good understanding of this coupled to well organised channels can help you double or triple your earnings in my experience.
[edited by: trillianjedi at 5:56 pm (utc) on July 23, 2005]