Ok, this is probably my being stupid...but I like to position a footer at the bottom of the page and I usually use a table with the height set to 100% and then the bottom row contains the footer info. Well, if you have:
<!
DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
at the top of the document, it appears to ignore the height setting...so I usually remove it so my table works like I want. It appears that removing this is what is causing my problems with Ajax controls.