closed as off topic by Roger Attrill, Marielle, DA01, Jitendra Vyas, Todd Sieling Sep 9 '11 at 6:46
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For iPhone, at least, this question has already been answered on StackOverflow: How do I lock the orientation to portrait mode in a iPhone Web Application?. The short answer is no, but that you can supply different CSS styles for each orientation, so that you can make it functional for both cases.
I'd be interested if there are any differences for Android/WP7/BB, if anyone knows, though.