I recently signed up for an unlimited data plan with Cingular.  The verdict is still out on that, but one thing I've noticed is that sidebar position, or more accurately content position, is quite important for websites.  Websites with sidebars on the left (and unnecessary content at the top) are difficult to navigate through to get to the actual content of the site.  But sidebars on the right are usually easily formatted by a mobile browser's "Optimized" mode, which places all the sidebar contents at the end.  This is an important thing if you want your website's content to be easily accessible by people with mobile devices.  Or you can use something like a mobile browser plugin to detect a mobile device and format the content accordingly.