As I mentioned in this previous post, in the HTML code example I used this META tag "<meta name='viewport' content='initial-scale=1.0' />".
I learned that from Chapter 24 "Web Views" of book "Programming iOS4". The author says according to "Safari Web Content Guide" "if no viewport is specified, the viewport can change when the app rotates. Setting the initial-scale causes the viewport size to adopt correct values in both orientations."
I searched the net and found This article from Safari Web Content Guide which provides quite a lot of information and you should have a look to learn more about "viewport".