How do you force an iPhone web app (not a native app) into landscape mode?
I don't know how to force it on startup, but I found a neat way to make your content scale correctly when changing from Portrait to Landscape, especially if you're using canvas.
<head>
<meta name="viewport" content="initial-scale=1.6; maximum-scale=1.0; width=device-width; "/>
...
</head>
So, as soon as your user flips the phone to landscape mode, your page will appear in the correct scale.