Cloud Four has an article on iPad orientation detection through CSS here:
http://www.cloudfour.com/ipad-orientation-css/
They supply an example of their techniques as well:
http://www.cloudfour.com/ipad-css/

NCZOnline – iPad web development tips:
Amongst other topics covered this article deals with the all important: User-agent string, iPad detection through Javascript as well as ways to ascertain screen dimensions and orientation:
http://www.nczonline.net/blog/2010/04/06/ipad-web-development-tips/

Smashing Magazine has porovided some well thought out issues to consider when starting out developing web-pages for the iPad & iPhone:
http://www.smashingmagazine.com/2010/05/28/web-development-for-the-iphone-and-ipad-getting-started/

HTMLGoodies has an article detailing more in-depth approaches to orientation on the iPad & iPhone:
Detect and Set the iPhone/iPad’s Viewport/Orientation Using JavaScript, CSS & MetaTags

Using CSS to handle Orientation on the iPad:
So, the upside is that this is a lot easier than you might think. WebKit/Safari supports the CSS media query declarations based on orientation. You can simply specify two CSS files which contain the layouts for portrait and landscape and place the following two lines in your pages HEAD tag.

<link rel="stylesheet" media="all and (orientation:portrait)" href="portrait.css">
<link rel="stylesheet" media="all and (orientation:landscape)" href="landscape.css">

And then simply amend the layout for each orientation accordingly, in the example below they use it to hide/show a H1 tag accordingly. Here’s a working example:
http://www.cloudfour.com/ipad-css/.