Can I just mention that at least on my phone, websites that "adapt" to landscape mode often bug the hell out of me. With my getting-old-eyesight, I often rotate to landscape mode to get bigger text so I can read it more easily. When a website goes "Oh look, more horizontal pixels, I'll shrink the middle column and add some nav on the left and ads on the right!"; I'll usually just hit [back] and move on to some other site.
(Possibly interesting idea: add some analytics so that you can detect portrait/landscape orientation changes, and report on bounce rates immediately afterwards…)
In our defence, we do our best to keep the standard zoom gesture working and we use a reasonable default font size. We're not 21 any more either. ;-)
However, for most of our users, a zoomed single-column layout is going to be less useful than switching to two columns when in landscape mode on an iPad.
On an iPhone, you should see a single-column layout in either orientation using the design we're currently testing.
(Possibly interesting idea: add some analytics so that you can detect portrait/landscape orientation changes, and report on bounce rates immediately afterwards…)