Lots of people choose to put ads on their site by first having a div as a placeholder in the normal span of the page, and then having another div later containing the AdSense code positioned where the original div was. The thinking there is to make sure the content comes up as easily as possible when a search engine reads the page and not get tangled up in extra details.
<body> <div id = "main_ad_container"></div> blah blah <div id = "main_ad_content"> <!-- adsense code --> </div> </body>
Imagine though that "blah blah" has a lot more to it and again, div "main_ad_content"> is positioned through css to appear where the original div is/was. I'm not sure if this method improves rankings or whatnot though.
Putting your most important ad code in a div will encourage the placement of the higher paying ads to it since it gives more importance to that ad block as being first even though it may be at the bottom of the page.